open my $fh1, 'first.csv';
my @first = <$fh1>;
close $fh1;

open my $fh2, 'second.csv';
my @second = <$fh1>;
close $fh2;

FIRST:
foreach my $line (@first) {
    foreach my $otherline (@second) {
        if ( $line eq $otherline ) {
            print "Found a duplicate!\n";
            next FIRST;
        }
    }
}