Far fetched but you can execute perl code in BOTH a regex-match (e.g. /HERE/) and the match-side of a regex substitution (e.g. s/HERE/../) and use its return value to shape the final regex pattern (see (??{ code })). So, using (??{ code }), you can do:

use constant APOSTROPHE =>  "\xE2\x80\x99" ;
my $string = "...";
$string =~ s/(?{{ APOSTROPHE }})/'/ ;
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If you want to call perl code in the replacement side of a substitution (e.g. s/../HERE/) simply add the /e (for eval) regex modifier, i.e. $string =~ s/.../APOSTROPHE/e

Perl code executes within the baby-cart operator suggested by haukex and also in choroba's ref-deref trick.

Edit: OP's code my $string =~ s/...//; has been modified above, thanks choroba.

bw, bliako