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'
}