Do you mean something along the lines of:

  use Config;
  BEGIN {
    die "You are socketless!\n"
      unless Config::config_vars(qw/d_socket/) eq 'define';
  }
[download]

If you need really specific socket capabilities there is any number of fine-grained tests you could perform. Randomly cutting and pasting from perldoc Config gives things like

=over
     `d_getsbyname'
         From d_getsrvby.U:

         This variable conditionally defines the
         `HAS_GETSERVBYNAME' symbol, which indicates to the C
         program that the getservbyname() routine is available to
         look up services by their name.
=cut
[download]

Is this what you were looking for?

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g r i n d e r