#!/usr/bin/perl
#
# only-in
# find lines which are in the first file, but not in the second.
#
use warnings;
use strict;

my ($input_file, $exclude_file) = (shift, shift);

die "Usage: $0 INPUT EXCLUDE\n" if ! $exclude_file;

my %exclude;

open (my $exclude_fh, $exclude_file ) ||
    die "Can't read exclude file '$exclude_file': $!\n";
while (defined(my $exclude = <$exclude_fh>)) {
    $exclude{$exclude} = 1;
}
close $exclude_fh;

open (my $input_fh, $input_file) ||
    die "Can't read input file '$input_file': $!\n";
while (defined(my $input = <$input_fh>)) {
    print $input if ! $exclude{$input};
}
close $input_fh;

##</code><code>##

$ cat file1
$i++;
$ only-in file1 file2
Nested quantifiers in regex; marked by <-- HERE in m/^$i++ <-- HERE ;
$/ at /home/qmechix/only-in line 27.