The lookbehind you show is for the string "!-". Looks like you might be confusing HTML comments for Perl ones.

I don't think you need a lookbehind.   m,^[^#]*(use|require)\s+([\w:]+)\s*[^;]*;,igm should do the trick. I added the 'm' modifier so that ^ would match inside of a string, and removed the 's' modifier so that '.' wouldn't match \n.

Note that won't correctly handle   require 5.003; If that matters, the change is left as an exercise.

Negative look-behinds have to be fixed-width so you can't use them here because the number of characters in front of "use|require" before you hit the "#" that you don't want to match isn't going to be constant.

I don't understand why you want to match "use++this" so perhaps you want to replace [\s\+]+ with simply \s+ ?

I don't see a lot of point in matching \s*[^;]*;. You can certainly shorten it to [^;]*; or .*?; (the second form can be a problem if you later add to the end of the regex). But I'd probably just drop it and allow for the ";" being dropped for various reasons.

The suggestion of /^[^#]*(use|require)... is pretty good but I'd at least add \b and prohibit quoted strings in in front: /^[^#'"q]*\b(use|require)... I'd might even just assume nothing but whitespace in front /^\s*(use|require)... but worry that someone might write BEGIN { require... for some reason.

Note also that you can require "config.pl" or even require $x, but those probably aren't the type of stuff you want to report anyway. (:

2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$
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-- Randal L. Schwartz, Perl hacker

If it helps, here's my test program. Still haven't found a solution to get all of them, but your comments are helping... Thanks!

Update: I added some more test cases...

#!/usr/bin/perl -w
use strict;

local $/ = undef;
my $file = <DATA>;

while ( $file =~

    # m,(use|require)[\s\+]+([\w:]+)\s*[^;]*;,igs
    # m,(?:use|require)\s+([\w:]+)\s*[^;]*;,igs
    # m,^[^#]*(use|require)\s+([\w:]+)\s*[^;]*;,igm 
     m,^(?:use|require)\s+([\w:.]+)\s*[^;]*;,gms

)
{ printf( "%-20s %-20s %-20s\n", $1 || '*', $2 || '*', $3 || '*' ); }

__DATA__

use one;

require two;

use
    three;

# use four;

# this is why you should use five.

use Six::Maybe;

require Seven.As.Well;

require Eight ; # haha!

use
  # nine; just kidding
  Nine;
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"We're experiencing some Godzilla-related turbulence..."

if ($string !~ /^#/) {
  print "Houston, we have an uncommented line \n";
}
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