Two ways spring to mind. First, connect to the port yourself to see if anything is listening (untested):

use IO::Socket::INET;
$sock=IO::Socket::INET->new(PeerAddr => 'machinename',
     PeerPort => $portnumber,
     Proto => 'tcp');
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And if you get a "connection refused" there's probably no listener.

Another way is to bind the port yourself. If it's a low-numbered port, you'll have to be root.

use IO::Socket::INET;
$sock=IO::Socket::INET->new(Listen => 1,
    LocalAddr => $interfaceaddres,
    LocalPort => $portnumber,
    ReuseAddr => 1,
    Proto => 'tcp');
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If you don't get a "port already bound" error, then it's not bound.

If either of these fail, $sock will be undef and $! will have your error message.