Well you could avoid the temporary variable by using //g in a LIST context instead. I wouldn't recommend it if the temporary list was likely to be of non-trivial size.

for ( $_[0]->seq() =~ /CG/g ) { ++$CG }
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If course if the body of your loop is really just ++$CG then all you are doing is counting the number of elements in the list. In that case the "pimply-goatse-operator" mentioned earlier in ths thread is what you want.