A couple of quibbles to start:

• Please surround your code samples with <code>..>/code< tokens; this makes it easier to read
• Please do not use $a and $b as variable names; these are special

When I run you first case through the debugger, I get 'empty array' as the result of
$x =&nbsp;(qq/$a/ =~ qq/$b/)
I suspect that this is a more appropriate way of writing your use of the regex:

($n = $a) =~ /$b/;
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With the proviso (I'm repeating myself) that $a and $b should not be used as variable names except in sort { $a <=> $b} @array;

Reparented from duplicate thread Reaped: Matching quoted variables. Cant make it work.. by Arunbear