open (F1,"f1.txt");
open (F2,"f2.txt");

while (<F1>){
    $HASH_a{$_}++; ### two hashes
}
close(F1);

while (<F2>){
    $HASH_b{$_}++;
### two hashes used to compare unique sets
}

foreach $line (sort keys %HASH_a){
    print "$line" if ( exists $HASH_b{$line} );
### switched this to check for existence of a key
### in one hash which we know exists in the other.
### also removed the \n in the print, because we
### never chomp()ed them off the originals.
}
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Update: Got rid of a couple of useless lines which tye pointed out to me in the CB.

Christopher E. Stith