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;
}