... your map qr{...} join ... irritated me a bit, because the processed list has only one element.

Yeah, that gets to me a bit too, whenever I use it. But that syntax is used in haukex's original article, so I'm willing to consider it an "idiom." :)

The important point is that the regex elements be somehow converted into a regex object. It's at this stage that any necessary boundary assertions are added. The only reasonable alternative I can see is something like

my $rx_search =
    join ' | ',
    map  quotemeta,
    reverse sort
    keys %replace
    ;
$rx_search = qr{ ... $rx_search ... }xms;
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