If your intent is to start an interactive shell on the server, then you'll probably want to spawn it with a TTY (using IO::Pty).

my $pty = new IO::Pty;
$pty->set_raw();
my $pid = fork // die;
unless ($pid) {
  $pty->make_slave_controlling_terminal();
  close STDIN; open STDIN, "+>&", $pty->slave;
  # ditto for STDOUT, ...; close $pty->slave;
  exit !exec @the_program;
}
close $pty->slave;
event_loop_passing_data_between($pty, $socket)
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To pass window size/ttype/etc., the Net::Telnet interface might be chosen. You've essentially reimplemented the telnetd.

OTOH, if you simply want to avoid signals from the child interrupting the server process, the following might be of use:

use POSIX;
my $pid = fork();
if ( $pid == 0 )
{
    setsid();
    close STDIN; close STDOUT; close STDERR;
    RunCommand( ... );
}
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Child setsid() detaches it from the controlling tty.