$myhash{REGEXP} = qr/^something$/;
print $myhash{REGEXP};  # -->  (?-xism:^something$)

$myhash{REGEXP} = qr/(?-xism:^something$)/;
print $myhash{REGEXP};  # -->  (?-xism:(?-xism:^something$))

$myhash{REGEXP} = qr/(?-xism:(?-xism:^something$))/;
print $myhash{REGEXP};  # -->  (?-xism:(?-xism:(?-xism:^something$)))

...