You'll need a // operator there, otherwise it eats the 0's in $b...

An alternative: 

$b = reverse $b;
$a =~ s{.\K}{chop $b}sge;
print $a;

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In reply to Re: The Regex Approach: Merging 2 strings like a zip by Anonymous Monk
in thread Merge 2 strings like a zip by tel2

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