First off a few comments not related to your question:
If your using any other regexs in your program avoid $& wrap your test in a () instead. $& will cause all regexs to run slower through your whole program.
Don't use map in a void context, it throws away the output. Try for (keys(${$args})) {$string .= $_ . "|"} I would usualy consider join even better but it would not work for my idea bellow.

As for you question you could try (?{ CODE }) to set a variable to the matched code, maybe something like this: 
our $matched_key; #global to hold the key that matched, off the top of
+ my head I don't think a 'my' variable would work.
for (keys($$args)) {
    $string .= "$_(?{$matched_key = $_})|";
}

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In reply to Re: Returning regexp pattern that was used to match by Ven'Tatsu
in thread Returning regexp pattern that was used to match by crabbdean

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