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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In reply to Re^2: Strange "undefined value as an ARRAY reference" error by ikegami
in thread Strange "undefined value as an ARRAY reference" error by johngg

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