>perl -wMstrict -le
"use constant RX => qr{ [atc]{3} }xms;
 my $s = 'the cat in the hat';
 print 'found' if $s =~ /${+RX}/;
 print qq{found a $1} if $s =~ /(${+RX})/;
"
Use of uninitialized value in regexp compilation ...
found
Use of uninitialized value in concatenation (.) or string ...
found a