my $search_for = join '|', map quotemeta, keys %replace;
...
s/\$($search_for)/$replace{$1}/g;
[download]

Additionally:
when doing things like this and you have two hash keys "foo" and "foobar", either use \b in the regex if possible, or sort the hash keys for length before joining. otherwise $foobar could be replaced with the value of $replace{foo}.
s/\$($search_for)\b/$replace{$1}/g;