#!/usr/bin/perl

sub s1 {
    my $t = shift;
    if ($t > 5) {
        print 5;
    } else {
        print $t;
    }
}

my $c = *main::s1{CODE};

*main::s1 = sub {
    print "extra";
    &$c(@_);
};

s1(1);
s1(7);