Hm, It seems to work if you use locale:

perl -we 'use POSIX; use locale; setlocale LC_NUMERIC, "fr_FR"; $x = "
+123,45"; $x+=0.1; print $x; setlocale LC_NUMERIC, ""; $x += 0.1; prin
+t $x;'
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Update: this does not work:

perl -we 'use POSIX; use locale; setlocale LC_NUMERIC, "fr_FR"; $x = "
+123,45";  print $x; setlocale LC_NUMERIC, ""; $x += 0.1; print $x;'
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So what I do now is to add 0 if it looks_like_number() and everything works fine. At least what I've tested until now...