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perlcompile - Introduction to the Perl Compiler-Translator


Perl has always had a compiler: your source is compiled into an internal form (a parse tree) which is then optimized before being run. Since version 5.005, Perl has shipped with a module capable of inspecting the optimized parse tree (B), and this has been used to write many useful utilities, including a module that lets you turn your Perl into C source code that can be compiled into a native executable.

The B module provides access to the parse tree, and other modules ("backends") do things with the tree. Some write it out as bytecode, C source code, or a semi-human-readable text. Another traverses the parse tree to build a cross-reference of which subroutines, formats, and variables are used where. Another checks your code for dubious constructs. Yet another backend dumps the parse tree back out as Perl source, acting as a source code beautifier or deobfuscator.

Because its original purpose was to be a way to produce C code corresponding to a Perl program, and in turn a native executable, the B module and its associated backends are known as "the compiler", even though they don't really compile anything. Different parts of the compiler are more accurately a "translator", or an "inspector", but people want Perl to have a "compiler option" not an "inspector gadget". What can you do?

This document covers the use of the Perl compiler: which modules it comprises, how to use the most important of the backend modules, what problems there are, and how to work around them.


The compiler backends are in the B:: hierarchy, and the front-end (the module that you, the user of the compiler, will sometimes interact with) is the O module. Some backends (e.g., B::C) have programs (e.g., perlcc) to hide the modules' complexity.

Here are the important backends to know about, with their status expressed as a number from 0 (outline for later implementation) to 10 (if there's a bug in it, we're very surprised):


Stores the parse tree in a machine-independent format, suitable for later reloading through the ByteLoader module. Status: 5 (some things work, some things don't, some things are untested).


Creates a C source file containing code to rebuild the parse tree and resume the interpreter. Status: 6 (many things work adequately, including programs using Tk).


Creates a C source file corresponding to the run time code path in the parse tree. This is the closest to a Perl-to-C translator there is, but the code it generates is almost incomprehensible because it translates the parse tree into a giant switch structure that manipulates Perl structures. Eventual goal is to reduce (given sufficient type information in the Perl program) some of the Perl data structure manipulations into manipulations of C-level ints, floats, etc. Status: 5 (some things work, including uncomplicated Tk examples).


Complains if it finds dubious constructs in your source code. Status: 6 (it works adequately, but only has a very limited number of areas that it checks).


Recreates the Perl source, making an attempt to format it coherently. Status: 8 (it works nicely, but a few obscure things are missing).


Reports on the declaration and use of subroutines and variables. Status: 8 (it works nicely, but still has a few lingering bugs).

Using The Backends ^

The following sections describe how to use the various compiler back ends. They're presented roughly in order of maturity, so that the most stable and proven backends are described first, and the most experimental and incomplete backends are described last.

The O module automatically enabled the -c flag to Perl, which prevents Perl from executing your code once it has been compiled. This is why all the backends print:

  myperlprogram syntax OK

before producing any other output.

The Cross Referencing Backend

The cross referencing backend (B::Xref) produces a report on your program, breaking down declarations and uses of subroutines and variables (and formats) by file and subroutine. For instance, here's part of the report from the pod2man program that comes with Perl:

  Subroutine clear_noremap
    Package (lexical)
      $ready_to_print   i1069, 1079
    Package main
      $&                1086
      $.                1086
      $0                1086
      $1                1087
      $2                1085, 1085
      $3                1085, 1085
      $ARGV             1086
      %HTML_Escapes     1085, 1085

This shows the variables used in the subroutine clear_noremap. The variable $ready_to_print is a my() (lexical) variable, introduced (first declared with my()) on line 1069, and used on line 1079. The variable $& from the main package is used on 1086, and so on.

A line number may be prefixed by a single letter:


Lexical variable introduced (declared with my()) for the first time.


Subroutine or method call.


Subroutine defined.


Format defined.

The most useful option the cross referencer has is to save the report to a separate file. For instance, to save the report on myperlprogram to the file report:

  $ perl -MO=Xref,-oreport myperlprogram

The Decompiling Backend

The Deparse backend turns your Perl source back into Perl source. It can reformat along the way, making it useful as a de-obfuscator. The most basic way to use it is:

  $ perl -MO=Deparse myperlprogram

You'll notice immediately that Perl has no idea of how to paragraph your code. You'll have to separate chunks of code from each other with newlines by hand. However, watch what it will do with one-liners:

  $ perl -MO=Deparse -e '$op=shift||die "usage: $0
  code [...]";chomp(@ARGV=<>)unless@ARGV; for(@ARGV){$was=$_;eval$op;
  die$@ if$@; rename$was,$_ unless$was eq $_}'
  -e syntax OK
  $op = shift @ARGV || die("usage: $0 code [...]");
  chomp(@ARGV = <ARGV>) unless @ARGV;
  foreach $_ (@ARGV) {
      $was = $_;
      eval $op;
      die $@ if $@;
      rename $was, $_ unless $was eq $_;

The decompiler has several options for the code it generates. For instance, you can set the size of each indent from 4 (as above) to 2 with:

  $ perl -MO=Deparse,-si2 myperlprogram

The -p option adds parentheses where normally they are omitted:

  $ perl -MO=Deparse -e 'print "Hello, world\n"'
  -e syntax OK
  print "Hello, world\n";
  $ perl -MO=Deparse,-p -e 'print "Hello, world\n"'
  -e syntax OK
  print("Hello, world\n");

See B::Deparse for more information on the formatting options.

The Lint Backend

The lint backend (B::Lint) inspects programs for poor style. One programmer's bad style is another programmer's useful tool, so options let you select what is complained about.

To run the style checker across your source code:

  $ perl -MO=Lint myperlprogram

To disable context checks and undefined subroutines:

  $ perl -MO=Lint,-context,-undefined-subs myperlprogram

See B::Lint for information on the options.

The Simple C Backend

This module saves the internal compiled state of your Perl program to a C source file, which can be turned into a native executable for that particular platform using a C compiler. The resulting program links against the Perl interpreter library, so it will not save you disk space (unless you build Perl with a shared library) or program size. It may, however, save you startup time.

The perlcc tool generates such executables by default.


For more information, see perlcc and B::C.

The Bytecode Backend

This backend is only useful if you also have a way to load and execute the bytecode that it produces. The ByteLoader module provides this functionality.

To turn a Perl program into executable byte code, you can use perlcc with the -B switch:

  perlcc -B

The byte code is machine independent, so once you have a compiled module or program, it is as portable as Perl source (assuming that the user of the module or program has a modern-enough Perl interpreter to decode the byte code).

See B::Bytecode for information on options to control the optimization and nature of the code generated by the Bytecode module.

The Optimized C Backend

The optimized C backend will turn your Perl program's run time code-path into an equivalent (but optimized) C program that manipulates the Perl data structures directly. The program will still link against the Perl interpreter library, to allow for eval(), s///e, require, etc.

The perlcc tool generates such executables when using the -O switch. To compile a Perl program (ending in .pl or .p):

  perlcc -O

To produce a shared library from a Perl module (ending in .pm):

  perlcc -O

For more information, see perlcc and B::CC.

Module List for the Compiler Suite ^


This module is the introspective ("reflective" in Java terms) module, which allows a Perl program to inspect its innards. The backend modules all use this module to gain access to the compiled parse tree. You, the user of a backend module, will not need to interact with B.


This module is the front-end to the compiler's backends. Normally called something like this:

  $ perl -MO=Deparse myperlprogram

This is like saying use O 'Deparse' in your Perl program.


This module is used by the B::Assembler module, which is in turn used by the B::Bytecode module, which stores a parse-tree as bytecode for later loading. It's not a backend itself, but rather a component of a backend.


This module turns a parse-tree into data suitable for storing and later decoding back into a parse-tree. It's not a backend itself, but rather a component of a backend. It's used by the assemble program that produces .plc bytecode.


This module is used by the B::CC backend. It walks "basic blocks". A basic block is a series of operations which is known to execute from start to finish, with no possibility of branching or halting or jumps into inner ops.


This module is a backend that generates bytecode from a program's parse tree. This bytecode is written to a .plc file, from where it can later be reconstructed back into a parse tree. The goal is to do the expensive program compilation once, save the interpreter's state into a file, and then restore the state from the file when the program is to be executed. See "The Bytecode Backend" for details about usage.


This module writes out C code corresponding to the parse tree and other interpreter internal structures. You compile the corresponding C file, and get an executable file that will restore the internal structures and the Perl interpreter will begin running the program. See "The Simple C Backend" for details about usage.


This module writes out C code corresponding to your program's operations. Unlike the B::C module, which merely stores the interpreter and its state in a C program, the B::CC module makes a C program that does not involve the interpreter. As a consequence, programs translated into C by B::CC can execute faster than normal interpreted programs. See "The Optimized C Backend" for details about usage.


This module prints a concise (but complete) version of the Perl parse tree. Its output is more customizable than the one of B::Terse or B::Debug (and it can emulate them). This module useful for people who are writing their own backend, or who are learning about the Perl internals. It's not useful to the average programmer.


This module dumps the Perl parse tree in verbose detail to STDOUT. It's useful for people who are writing their own backend, or who are learning about the Perl internals. It's not useful to the average programmer.


This module produces Perl source code from the compiled parse tree. It is useful in debugging and deconstructing other people's code, also as a pretty-printer for your own source. See "The Decompiling Backend" for details about usage.


This module decodes .plc bytecode back into a readable parse-tree, the reverse of the "B::Assembler". It's not a backend itself, but rather a component of a backend. It's used by the disassemble program that produces bytecode.


This module inspects the compiled form of your source code for things which, while some people frown on them, aren't necessarily bad enough to justify a warning. For instance, use of an array in scalar context without explicitly saying scalar(@array) is something that Lint can identify. See "The Lint Backend" for details about usage.


This module prints out the my() variables used in a function or a file. To get a list of the my() variables used in the subroutine mysub() defined in the file myperlprogram:

  $ perl -MO=Showlex,mysub myperlprogram

To get a list of the my() variables used in the file myperlprogram:

  $ perl -MO=Showlex myperlprogram



This module prints the contents of the parse tree, but without as much information as "B::Debug". For comparison, print "Hello, world." produced 96 lines of output from B::Debug, but only 6 from B::Terse.

This module is useful for people who are writing their own backend, or who are learning about the Perl internals. It's not useful to the average programmer.


This module prints a report on where the variables, subroutines, and formats are defined and used within a program and the modules it loads. See "The Cross Referencing Backend" for details about usage.


BEGIN{} blocks are executed while compiling your code. Any external state that is initialized in BEGIN{}, such as opening files, initiating database connections etc., do not behave properly. To work around this, Perl has an INIT{} block that corresponds to code being executed before your program begins running but after your program has finished being compiled. Execution order: BEGIN{}, (possible save of state through compiler back-end), INIT{}, program runs, END{}.

CC backend: goto, sort with non-default comparison. last for non-loop blocks.

improve XSUB handling (both static and dynamic)

sv_magic can do SvREFCNT_inc(obj) which messes up precalculated refcounts

allocation of XPV[INAHC]V structures needs fixing: Perl tries to free them, whereas the compiler expects them to be linked to a xpv[inahc]v_root

list the same as X[IPR]V structures.

ref counts

perl_parse replacement

fix cstring for long strings

compile-time initialisation of AvARRAYs

signed/unsigned problems with NV (and IV?) initialisation and elsewhere?

CvOUTSIDE for ordinary subs

DATA filehandle for standalone Bytecode program (easy)

DATA filehandle for multiple bytecode-compiled modules (harder)

DATA filehandle for C-compiled program (yet harder)



This document was originally written by Nathan Torkington, and was maintained by the perl5-porters mailing list up to Perl version 5.8.

This version with all the compiler options is now part of the B::C compiler module, maintained by Reini Urban


perlguts with a simplier version at

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