$p="horraen=sumsel\[13\]\)\n";
 $m="sumsel\[13\]";
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The above are exactly equivalent to

 $p="horraen=sumsel[13])\n";
 $m="sumsel[13]";
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If you really want backslashes in the strings you need to double them or use single quotes.

 $p="horraen=sumsel\\[13\\]\\)\n";
 $m="sumsel\\[13\\]";
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 $p='horraen=sumsel\[13\]\)' . "\n";
 $m='sumsel\[13\]';
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Actually since $m is a regex it's more ideomatic to use qr// unless there's a reason you can't. Also I don't think you really wanted backslashes in $p.

 $p="horraen=sumsel[13])\n";
 $m=qr/sumsel\[13\]/;
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