Also, you can use \Q...\E to quote otherwise regex-meaningful characters. E.g.

$p = "horraen=sumsel[13])\n";  # no backslashes required
$m = qr/\Qsumsel[13]\E/;  # actually in this case qr/\Qsumsel[13]/ is 
+enough
print $p;
if ( $p =~ /$m\)/ ) { print "yeah!\n"; }
else { print "why isn't it matching?\n"; }
__END__
horraen=sumsel[13])
yeah!
[download]

See also quotemeta.