As philcrow says, this works for me too.

But I wanted to point out something else ...

Is there a good reason that you're using the "evaluate" flag? (the e of ge)  Because, if not, you may just have wanted:

$string =~ s/$regex/'bar'/g;

# This will print ... "this is a 'bar'"
# but with "/ge" .... "this is a bar"
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It took me a moment to figure out why the output didn't contain apostrophes!