Put brackets around the hash element to explicitly interpolate:

$instr =~ s|$pat|${patt1{$pat}}|egs;

Update: sorry, brainfreeze, this will not work

2nd Update:This node is getting upvoted even though I marked it as wrong, so I thought I'd clarify:This advice is wrong and won't work, please don't use it, it was conceived and written in an inexplicable moment of fuzzy-headedness. runrig's solution below works perfectly.

In reply to Re: interpolation in replacement part by tirwhan
in thread interpolation in replacement part by anniyan

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