$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!