I've never had a the need to use a lookaround, but your question piqued my curiosity. As an experiment, I used Regexp::Assemble to rework your example because it just does the right thing with assertions, lookaheads, and lookbehinds.

#! /usr/bin/perl -slw

use strict;
use Regexp::Assemble;

my $needs = Regexp::Assemble->new->add( qw[ FILE INFO ] );
my $has   = Regexp::Assemble->new->add( qw[ FILE INFO SYSLOG ] );

while( defined( $_ = <DATA> )) {
    chomp;
    if( /($needs)/ ) {
        print $needs->as_string;
    }
}
while( defined( $_ = <DATA> )) {
    chomp;
    if( /($has)/ ) {
        print $has->as_string
    }
}

__DATA__
FILE
INFO
SYSLOG
[download]

I believe, if I"ve understood you correctly, that it works the way that it should. What do you think?

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