Backslash-backreferences aren't cricket in the replacement portion of s///. You would want '$1bar', and then you'd need to use the /ee flag to get it to eval properly: once to turn $bar into $1foo and once to sub for $1. But $1foo isn't a proper expression, you need to put a . to concatenate them, or double-quotes around it:

$replace = '$1.bar'; # or '"$1bar"'
s/$find/$replace/eeg;
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Update: It might be more secure to put the quotes in the s/// expression itself:

$replace = 'warn "AARGH!"';
s/$find/qq(qq($replace))/eeg;
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