You need to use a string eval, and make sure to escape the dollar-sign on "$mystring":

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

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

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

print "$mystring\n";
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That is: $string1 and $string2 will be interpolated before being passed to eval, and $mystring will not be interpolated. In the example above, eval will see:

$mystring =~ s/sometext(.*)something(.*)/$1:$2/;
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and that is what will be executed within the eval.