I don't know whether this is better or not but I tend to use the quotishness of compiled regexen with a localized list separator rather than join.

my $rxPattern;
{
    local $" = q{|};
    $rxPattern = qr{(?x)
       (@{ [ map { quotemeta } keys %map ] })
       };
}
$string =~ s{$rxPattern}{$map{$1}}g;
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I'd be interested to know if this is a sensible approach.

Cheers,

JohnGG