The OP is right. If you use a variable in a Perl regexp block, and that variable is defined outside the regexp, it's gotta be a package variable. Regexps acts as captures when they are compiled.

sub foo {
   my ($x) = @_;
  '' =~ /(?{ print "$x\n" })/;
} 

foo('a');
foo('b');
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Variable "$x" will not stay shared at (re_eval 1) line 2.
a
a
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The warning was added in 5.10, but the problem has always existed.

However, the OP should have localized the variables. Bonus, this initializes @common_letters to ().

   local our @common_letters;
   local our @wordsCopy = @words;
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