You are assigning a list (@_) to a scalar ($socket). Fixing this, cleaning up the indenting, removing the prototypes and connecting to a public service gives:

#!/usr/bin/env perl 
use strict;
use warnings;
use IO::Socket;

my $s = client_socket_create ();
client_main ($s);
exit;

sub client_socket_create {
    my $socket = IO::Socket::INET->new (
        PeerHost => 'www.perlmonks.org',
        PeerPort => 80,
        Proto    => 'tcp'
    );
    print "$socket \n";
    return $socket;
}

sub client_main {
    my ($socket) = @_;
    print "$socket \n";
}
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which prints out the same socket twice as one supposes is intended.