concatenate $FILENAME and .yyy

[Jennifer Pan]

Hello all, 

I stdin a file name: xxx.txt
and I would like to have a output file name called x.txt.yyy

This is how I do it 

$FILENAME = <STDIN>;
$output = "$FILENAME.y";
print "$output";


And the result is only
.yyy

How do I concatenate $FILENAME and .yyy? 

Thank you