#!/usr/bin/perl

use strict;
use warnings;

sub myTrim {
    my ($str) = @_;
    my ($trimmed) = ($str =~ /\s*(.*)\s*/);
    return $trimmed;
}

my @allTags = ();

foreach my $file (<[0-9][0-9][0-9][0-9].txt>) {

    my @fileTags = ();

    open(my $handle, '<', $file) or die 'couldn\'t open file';

    while (my $line = <$handle>) {
        if ($line =~ /^tags/) {
            my ($tagsString) = ($line =~ /^tags\s+(.*)/);
            @fileTags = (@fileTags, split(/,/, $tagsString));
        } 
    }

    close($handle);

    # some of the items in @fileTags disappear (not just duplicate items)
    my %uniqueTags = map { myTrim $_, 1 } @fileTags;
    @allTags = (@allTags, keys %uniqueTags);
}

print "@allTags\n";