The others have given much better advice but if you download the sources for perl you can see the code you say you want to modify. Hopefully this will convince you that you should listen to the wiser PerlMonks.

perl -ne 'print "$ARGV:$.:$_" if /substr/ .. /^}/' pp.c
[download]

The above will extract and print the substr function that is in the perl source file pp.c.