I would use something like Net::Ping or simply the external ping utility. Most of the external ping utilities have a way of only checking once or a short time whether a host is reachable, for example the Solaris and Win32 ping commands, so using that should be easy as well.

Pinging might not work well for you. Computers tend to be 'pingable' soon after they boot up, but other services such as a database or webserver might start considerably later. You might want to brute force it: just send your command and try to analyze the result and resend if necessary. Or perhaps figure out some other test that tells you that the service you need is running.

Cheers
Chris

use warnings;
use strict;
use Socket;
my ($remote,$port, $iaddr, $paddr, $proto, $line);

$remote  = $ARGV[0] || die "usage: $0 hostname";
$port    = 22 ;  # the SSH port
$iaddr   = inet_aton($remote) || die "no such host: $remote";
$paddr   = sockaddr_in($port, $iaddr);
$proto   = getprotobyname('tcp');

socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket() failed: $!
+";
print "Connecting to port 22...\n";
connect(SOCK, $paddr) || die "connect() failed: $!";
print "Connected.\n";
$line = <SOCK>;
print $line;
exit 0 if $line =~ /SSH/;
exit 1;
[download]

$ perl sshping.pl localhost; echo $?
Connecting to port 22...
Connected.
SSH-1.99-OpenSSH_x.xx Debian-xxx
0
[download]

$ perl sshping.pl localhost; echo $?
Connecting to port 22...
connect() failed: Connection refused at sshping.pl line 14.
111
[download]

$ if perl sshping.pl localhost; then echo up; else echo down; fi
Connecting to port 22...
connect() failed: Connection refused at sshping.pl line 14.
down
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