given a reference file, output every line from STDIN that does not appear in the reference file, preserving the order of the lines in STDIN.

cat-notin.pl

#!/usr/bin/perl

use strict;
use warnings;

my $in = shift || die "usage: $0 <ref-file>\n\n";

my %unwanted = %{&load_words($in)};

my $line;
while ($line=<>) {
    my $word;
    chomp($line);
    $line =~ s/^\s*//;
    $line =~ s/\s*$//;
    if ($line =~ /^\d+\s+(.*)\s*$/) {
  $word  = $1;
    }
    else {
	$word = $line;
    }
    print "$line\n" unless (defined($unwanted{$word}));
}

sub load_words {
    my $file = shift;
    my %words = ();
    open(IN, $file) or die "$0 error: $!";
    while (<IN>) {
	chomp;
	$words{$_} = 1;
    }
    close(IN);
    return \%words;
}