#!/usr/bin/env perl

use strict;
use warnings;

my $out_file = q{result.list};
my $search_re = qr{hello};
my %entry_seen = ();

print q{Enter reference filename: };
chomp(my $ref_file = <STDIN>);

open my $fh_ref, q{<}, $ref_file or die qq{Can't open $ref_file: $!};
open my $fh_out, q{>}, $out_file or die qq{Can't open $out_file: $!};

while (defined(my $txt_file = <$fh_ref>)) {
    chomp $txt_file;
    open my $fh_txt, q{<}, $txt_file or do {
        warn qq{! SKIPPING: $txt_file: $!};
        next;
    };  

    print qq{PROCESSING: $txt_file\n};

    while (defined(my $txt_line = <$fh_txt>)) {
        chomp $txt_line;

        next if $txt_line !~ $search_re;

        if (! $entry_seen{$txt_line}++) {
            print $fh_out qq{[$txt_file] $txt_line\n};
        }
    }

    close $fh_txt;
}

close $fh_out;
close $fh_ref;

##</code><code>##

ken@ganymede: ~/tmp/PM_DIFF
$ cat a.txt
b.txt
c.txt
dummy.txt
d.txt
ken@ganymede: ~/tmp/PM_DIFF
$ cat b.txt
Hello
hello
hello, world
goodbye
ken@ganymede: ~/tmp/PM_DIFF
$ cat c.txt
hello world
hullo
shellow
ken@ganymede: ~/tmp/PM_DIFF
$ cat d.txt
hello, world
hello and goodbye
Hello, world
ken@ganymede: ~/tmp/PM_DIFF
$ cat result.list
ken@ganymede: ~/tmp/PM_DIFF
$ pm_diff.pl
Enter reference filename: not_a_file
Can't open not_a_file: No such file or directory at ./pm_diff.pl line 13, <STDIN> line 1.
ken@ganymede: ~/tmp/PM_DIFF
$ cat result.list
ken@ganymede: ~/tmp/PM_DIFF
$ pm_diff.pl
Enter reference filename: a.txt
PROCESSING: b.txt
PROCESSING: c.txt
! SKIPPING: dummy.txt: No such file or directory at ./pm_diff.pl line 19, <$fh_ref> line 3.
PROCESSING: d.txt
ken@ganymede: ~/tmp/PM_DIFF
$ cat result.list
[b.txt] hello
[b.txt] hello, world
[c.txt] hello world
[c.txt] shellow
[d.txt] hello and goodbye
ken@ganymede: ~/tmp/PM_DIFF
$