There's no need to iterate over the keys of the hash to see if the acronym exists, just use the exists function!

chomp($in = <STDIN>);
if (exists $acro{$in}) {
   print "$in is the acronym for $acro{$in}\n";
} else {
   print "Sorry, unknown acronym"; 
}
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Also, the last part of your while loop where you're asking if the user wants to try again seems garbled.