Oh! Another to fix the problem is to add a no-op to the regexp. 

$\ = "\n";
$pat = 'b'; $_='abc'; s/$pat/!/g;     print;
$pat = '';  $_='abc'; s/$pat/@/g;     print;
$pat = '';  $_='abc'; s/(?:$pat)/#/g; print;

[download]

outputs 

a!c
a@c
#a#b#c#

[download]

In reply to Re^2: s/// with an empty pattern uses the previous pattern of a s/// by ikegami
in thread s/// with an empty pattern uses the previous pattern of a s/// by ckeith100

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