use strict;
use strict;
use warnings;

my %file2;
open my $file2, '<', '/tmp/dog.txt' or die "Couldn't open file2: $!";
while ( my $line = <$file2> ) {
    ++$file2{$line};
}

open my $file1, '<', '/tmp/cat.txt' or die "Couldn't open file1: $!";
while ( my $line = <$file1> ) {
    print $line if $file2{$line};
}