first problem is your assigning a scalar to an an array

Should be my $i = "l-12345.in.some.domain.com";

Second, your question seems a bit vague, do you know which two things you need to match?

If so you should be able to say

$i =~/($first)\.($second)/;
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Now $first is in $1 and $second is in $2 if they where found.

Of course this doesn't take into account boundaries.
If $first = "dog" and $second ="com" this will match fogdog.com as well. You need to determine what your boundaries are going to be.

-Lee

"To be civilized is to deny one's nature."