I am getting the error as "IO::Socket: Unknown error propagated in line4";

That error message did not come from the script I posted. (Line 4 is a blank line!)

I ran the script as "perl socket.pl 25000". Does this notify the same ?;

The port number will make no difference:

C:\test>start /b lsn.pl 25000

C:\test>Running with pid:12100

C:\test>lsn.pl 25000
Running with pid:11664
Only one usage of each socket address (protocol/network address/port) 
+is normally permitted at C:\test\lsn.pl line 7.

C:\test>taskkill /f /pid 12100
SUCCESS: The process with PID 12100 has been terminated.
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Code with line added to print pid:

#! perl -slw
use strict;
use IO::Socket;

print "Running with pid:$$";

my $lsn = IO::Socket::INET->new(
    Listen => 1, LocalPort => $ARGV[0]
) or die $^E;

while( my $client = $lsn->accept ) {
    close $client;
}
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