I thought that since 

'aa' =~ /(.)\1/

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there can be something like:

use 5.010;
my @a = qw/a d/;
$_ = '0a1d1y0z';
say s/(\d)${a[\1]}/xx/g;
say;

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And it'll say "2" and "xxxx1y0z", but of course it won't work.

I could capture each pair, and then decide on what to replace it with, in substitution part, with an "e" modifier. But I would like an "s" operator to return number of 'real' substitutions made.

I came up with 

say s/(\d)([a-z])(?(?{ $2 ne $a[$1] })(*FAIL))/xx/g;
say;

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But it doesn't look very efficient nor pretty. Is there a better solution?

(In practice, there's not an array, but HoA, with keys being sub-matches and indices depending on external conditions. And matches are not simple pairs of characters, and strings are a little longer.)

In reply to Using sub-matches to interpolate hash keys (array indices)? by vr

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