use Data::Dumper;

my %myhash;
$myhash{REGEXP} = qr/^something$/;

print Dumper \%myhash;

__END__
$VAR1 = {
          'REGEXP' => qr/(?-xism:^something$)/
        };

##</code><code>##

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

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

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

...

##</code><code>##

---
REGEXP: !!perl/regexp (?-xism:^something$)

##</code><code>##

package YAML::Type::regexp;
...
use constant _QR_TYPES => {
    '' => sub { qr{$_[0]} },
    x => sub { qr{$_[0]}x },
    i => sub { qr{$_[0]}i },
    ...
    msix => sub { qr{$_[0]}msix },
};

sub yaml_load {
    my $self = shift;
    my ($node, $class) = @_;
    return qr{$node} unless $node =~ /^\(\?([\-xism]*):(.*)\)\z/s;
    my ($flags, $re) = ($1, $2);
    $flags =~ s/-.*//;
    my $sub = _QR_TYPES->{$flags} || sub { qr{$_[0]} };
    my $qr = &$sub($re);
    bless $qr, $class if length $class;
    return $qr;
}

##</code><code>##

REGEXP: !!perl/regexp (?i-xsm:^something$)
                        |
                        v
         qr{^something$}i