Double interpolation of captured substrings

[bobf]

Oh wise ones,

I am trying to write a function that returns a string that is assembled from a regex, but the order that the captured substrings are assembled in is determined by one of the passed arguments. Here is an overly simplified example of what I'm trying to do:

my $newstring = munge_string( 'one_two_three', '312' );

sub munge_string
{
    my ( $string, $patternkey ) = @_;

    # patterns can also be 'text$1$2$3', etc
    my %patterns = ( 123 => '$1$2$3',
                     312 => '$3$1$2' );

    # in reality the regex is also dynamic (based on $patternkey)
    $string =~ m/(\w+)_(\w+)_(\w+)/;

    return $patterns{$patternkey}; # want 'threeonetwo', got '$3$1$2'
}

How can I get the function to return a double-interpolated value (once to get the hash value, then again to get the captured substrings)?

I tried using eval (to no avail), and then s///e instead of m//, but I only got single interpolation:

$string =~ s/(\w+)_(\w+)_(\w+)/$patterns{$patternkey}/e;
print $string; # '$3$1$2'

If I put the replacement pattern in directly, though, it works fine:

$string =~ s/(\w+)_(\w+)_(\w+)/$3$1$2/;
print $string; # 'threeonetwo'

or

$string =~ s/(\w+)_(\w+)_(\w+)/join( '', $3, $1, $2 )/e;
print $string; # 'threeonetwo'

Is there a way to do this cleanly, or am I going about this the wrong way? Thanks in advance.