Sounds like you forgot to use utf8::upgrade($from);

No, I tried this:

#!/usr/bin/perl

$Lang{$L}{"Repl"} = { "m" => "Monat" };  # mimic Date::Manip
$_ = "Mär";

print "before: $_\n";

foreach $from (keys %{ $Lang{$L}{"Repl"} }) {
  $to=$Lang{$L}{"Repl"}{$from};

  utf8::upgrade($from);  # Use Unicode semantics
  s/(^|[^[:alpha:]])$from($|[^[:alpha:]])/$1$to$2/i;
}    
print "after:  $_\n";
[download]

which prints

before: Mär
after:  Monatär
[download]

Using

  s/(^|[^\p{IsAlpha}])$from($|[^\p{IsAlpha}])/$1$to$2/i;
[download]

in place of the above substitution does work fine, though (as does adding use locale to the [:alpha:] version):

before: Mär
after:  Mär
[download]

presuming other changes will be made as well

No, using unicode semantics is enough.

I was referring to "MÄR" also working (in addition to "Mär") in the context of Date::Manip...  which it won't unless that abbreviation is also being set up in the respective $$d{"month_abb"}=...