you can use the ($result)= ($string=~ m/pattern/); idiom this way:

#!/usr/bin/perl -w
use strict;
while( my $string=<DATA>)
  { my( $domain)= ($string=~ m{(?:^|\.)       # the beginning of the s
+tring or .
                               ([^.]*)        # anything but . (and st
+ore it in $1)
                               \.com          # .com
                              (?:\/|$)       # a / or the end of the s
+tring
                              }x);
    print "domain: $domain\n";
  }

__DATA__
l-12345.in.some.domain.com
l-12345.in.some.domain.com/blargh/index.html
domain.com/blargh/index.html
domain.com
domain.com/
l-12345.in.some.domain.com/blargh/foo.com
nope.com.domain.com/blargh/foo.com
l-12345.in.some.domain.com/blargh/nope.foo.com
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