Here is a solution that detects when initial alphas of separate strings are the same regardless of case, and that I think is immune to differences in the locale or the Unicode character set that may be in effect. My understanding of the effects of locale and Unicode on regex behavior continues to be a bit vague.

I would be interested in any comments, links, etc., from other monks on my notions that:

1. The regex expression  [^\W\d_] matches an alpha character regardless of locale or the ASCII or Unicode character set in effect during compilation of the script.
2. The expressions  [[:alpha:]] and  [^\W\d_] have exactly the same behavior.
3. The  //o modifier of the  m{ \A ([^\W\d_]) }xmso regex prevents re-compilation of the regex on multiple calls and so is, in theory, more efficient.
4. After two strings in which the first characters are identical have been bitwise-xored in the expression  ((lc str1) ^ (lc str2)) =~ /^\0/, the regex  /^\0/ is not guaranteed always to match with the first character of the resultant string regardless of locale or character set.

>perl -wMstrict -le
"print '----- output -----';
 while (my ($word1, $word2) = splice @ARGV, 0, 2) {
   print qq{:$word1: and :$word2: initial letters },
         first_letter_matches($word1, $word2)
         ? 'same' : 'different';
   }
 sub first_letter_matches {
   my ($w1, $w2) = @_;
   return $w1 =~ m{ \A ([^\W\d_]) }xmso
       && $w2 =~ m{ \A     $1     }xmsi
        ;
   }
"
a a  a A  Ab abc  b b  abcd A  A1% a2#
"" ""  "" a  a ""  a b  a bc  ab c  123 123  %# %#
----- output -----
:a: and :a: initial letters same
:a: and :A: initial letters same
:Ab: and :abc: initial letters same
:b: and :b: initial letters same
:abcd: and :A: initial letters same
:A1%: and :a2#: initial letters same
:: and :: initial letters different
:: and :a: initial letters different
:a: and :: initial letters different
:a: and :b: initial letters different
:a: and :bc: initial letters different
:ab: and :c: initial letters different
:123: and :123: initial letters different
:%#: and :%#: initial letters different
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