$string1 = 'sometext(.*)something(.*)';
$string2 = '$1:$2';

$mystring = "sometext--blahblah--something--foobar--";

eval "\$mystring =~ s/$string1/$string2/";

print "$mystring\n";

##</code><code>##

$mystring =~ s/sometext(.*)something(.*)/$1:$2/;