I don't quite understand the distinction either, but locales come in here too.

I've done this quick check:

for l in C hu_HU de_DE; do echo $l:; LANG=$l perl -we 'use locale; for
+ $s ("\x{e1}", "\x{151}", "\x{a3}") { for $r (qr/\w/, qr/[[:alnum:]]/
+, qr/\pL/) { printf qq["\\x{%x}" %s /%s/\n],
ord($s), ($s =~ qr/$r/) ? "=~" : "!~", $r; } }'; done
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output is

C:
"\x{e1}" !~ /(?-xism:\w)/
"\x{e1}" !~ /(?-xism:[[:alnum:]])/
"\x{e1}" =~ /(?-xism:\pL)/
"\x{151}" =~ /(?-xism:\w)/ # why?
"\x{151}" =~ /(?-xism:[[:alnum:]])/ # why?
"\x{151}" =~ /(?-xism:\pL)/
"\x{a3}" !~ /(?-xism:\w)/
"\x{a3}" !~ /(?-xism:[[:alnum:]])/
"\x{a3}" !~ /(?-xism:\pL)/
hu_HU:
"\x{e1}" =~ /(?-xism:\w)/
"\x{e1}" =~ /(?-xism:[[:alnum:]])/
"\x{e1}" =~ /(?-xism:\pL)/
"\x{151}" =~ /(?-xism:\w)/
"\x{151}" =~ /(?-xism:[[:alnum:]])/
"\x{151}" =~ /(?-xism:\pL)/
"\x{a3}" =~ /(?-xism:\w)/
"\x{a3}" =~ /(?-xism:[[:alnum:]])/
"\x{a3}" !~ /(?-xism:\pL)/
de_DE:
"\x{e1}" =~ /(?-xism:\w)/
"\x{e1}" =~ /(?-xism:[[:alnum:]])/
"\x{e1}" =~ /(?-xism:\pL)/
"\x{151}" =~ /(?-xism:\w)/
"\x{151}" =~ /(?-xism:[[:alnum:]])/
"\x{151}" =~ /(?-xism:\pL)/
"\x{a3}" !~ /(?-xism:\w)/
"\x{a3}" !~ /(?-xism:[[:alnum:]])/
"\x{a3}" !~ /(?-xism:\pL)/
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With no locale;, you get the same output as with with the C locale.