Fixed:

#!/usr/bin/perl

use strict;
use warnings;

#declaration of variables
my $file1   = '/opt/programming/work/f1.txt';
my $file2   = '/opt/programming/work/f2.txt';
my $outfile = '/opt/programming/work/of.txt';

my %results;
open my $f1_fh, '<', $file1 or die "Could not open $file1: $!\n";
while (defined(my $line = <$f1_fh>)) {
    chomp $line;
    $results{$line} = ();
}
close $f1_fh;

open my $output_fh, '>', $outfile or die "Cannot open $outfile for wri
+ting: $!\n";
open my $f2_fh,     '<', $file2   or die "Could not open $file2: $!\n"
+;

while (defined(my $line = <$f2_fh>)) {
    chomp $line;

    if (not exists $results{$line}) {
        print {$output_fh} $line, "\n";
    }
}

close $f2_fh;
close $output_fh;
[download]

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