And it looks like the problem is not in IO

Correct. That's what my changes show.

I don't really understand this output very well, but it looks like it treats this regexes differently.

It's not a problem that /\w/ and /[\w]/ compile differently. It's a problem that they don't compile to something equivalent.

use locale;
utf8::upgrade( my $s = chr(0xC9) );   # e-acute
print "Outside char class: ", $s =~ m/\w/   ? "" : "no ", "match\n";
print "Inside char class:  ", $s =~ m/[\w]/ ? "" : "no ", "match\n";
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LANG=en_CA.utf8 perl test.pl
Outside char class: no match
Inside char class:  match
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