If you had tested it with more input, you'd have found it is wrong.

You wrote:

    foreach my $key (keys %convs)
    {
        $in =~ s/$convs{$key}/$key/g;
    }
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The OP wrote:

And that *almost* works perfectly. Where it doesn't work is if there's two of these special characters grouped around a normal character... [...] And since I can get these in any order, there's no way to put them in an order that always works.

Since you stored the patterns as values in a hash, the order in which they're substituted gets randomized such that with the input you tested with, the code happens to work.