use strict;
use warnings;

my (%h, $handle);

open($handle, 'file1.txt');
while (<$handle>) {
    # Creates hash using first field of each line as keys
    $h{(split /,/)[0]} = ();
}
close($handle);

open($handle, 'file2.txt');
while (<$handle>) {
    # Prints second field of each line if it exists in the hash
    chomp; print "$_\n" if exists $h{$_ = (split /,/)[1]};
}
close($handle);