If you want to compare all lines of the 2 files, just like the unix "diff" command (or "tkdiff"):

use warnings;
use strict;
open my $fh1, '<', 'file1.txt' or die "Can not open file1.txt $!\n";
open my $fh2, '<', 'file2.txt' or die "Can not open file2.txt $!\n";
while (<$fh1>) {
    chomp;
    my $line_f2 = <$fh2>;
    chomp $line_f2;
    if ($_ ne $line_f2) {
        print "Mismatch at line $.\n";
        print "file1: $_, file2: $line_f2\n";
    }
}
close $fh2;
close $fh1;
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