in reply to s///g within m//g regions

There are quite a few techniques you can use to make your regexen nicer, any combination of which might be useful to you, see for example /x, /e, /r, \K, and lookarounds in perlre. In the following example I've thrown all of them together into one, which may be overkill. (I see ikegami has posted something similar, albeit non-functional, while I was composing this.)

my $data = <<'END'; foo123bar # foo123bar foo456bar # xyz foo789bar abc foo456bar END $data =~ s{ ^ # beginning of line \h* \# \h* # comment lines \K # keep everything up to here in replacement (?<comment> \N*) # capture the comment $ # end of line } { handle_foobar( $+{comment} ) }msxge; sub handle_foobar { return shift =~ s{ foo \K (\d+) (?= bar ) }{ $1 =~ tr/0-9/a-j/r }msxger; }
I could turn the string into a line-list

Note it's also possible to open a string as an in-memory filehandle:

my $str = <<'END'; Hello # START World # END Aaa # START Bbb # END Ccc END open my $fh, '<', \$str or die $!; while (<$fh>) { chomp; if ( /^# START/ .. /^# END/ ) { print "<$_>\n"; } } close $fh;

Also, a more advanced regex technique is m/\G.../gc parsing, which is described in perlop. Also, as for your example here, if the delimiters need to be escaped, see Regexp::Common::delimited.