#! perl -slw
use strict;

sub munge { 
	my( $str, $regex ) = ( shift, shift );
	my @matches = ( undef, $str =~ $regex );
	return join'', @matches[ @_ ];
}

print munge( 
    'one_two_three', 
    '^([^_]+)_([^_]+)_([^_]+)$', 
    2, 1, 3 
);
__END__
[10:24:12.59] P:\test>junk
twoonethree