If you use locale, then the string comparision operators (and the default sort block too) will sort by the current locale, as given by LC_COLLATE. For example, this code

LC_ALL=hu_HU perl -wle 'use locale; print join " ", sort "op\xf3\xf5\x
+f6"=~/\S/g;'
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gives the correct order of the letters: o ó ö ő p.

Thus, instead of a regular expression such as m/^[A-Fa-f]/, you could use a comparision like $_ ge "g". I don't know how you can do this with regular expressions.

(Updated one typo.)