This example fails because it'll also return true if non-alphabetic characters match. You specified "letters".

sub first_character_matches {
  return( ((lc shift) ^ (lc shift)) =~ /^\0/ );
}
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And a command-line version to play with:

1) poerbook:~% perl -wle 'print "yes" if ( ((lc shift) ^ (lc shift)) =
+~ /^\0/ )' abc DBC
1) poerbook:~% perl -wle 'print "yes" if ( ((lc shift) ^ (lc shift)) =
+~ /^\0/ )' abc ABC
yes
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