=~ binds a scalar expression to a pattern match.  So you could do "@a" =~ ... (which would interpolate the array's contents into a string (space-separated)), or simply assign what the backticks return to a scalar variable (instead of an array) in the first place.

use warnings;

@hostnames = qw(goodhost badhost);

foreach (@hostnames){
    @a = `ping $_`;
    print "for $_ checking.....";
    if ("@a" =~ /please check/){
        print " For $_ : host not available.";
    }
check/;
}
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All I wanted to say: badhost : host not available But doesnt work

Maybe you want to check case-insensitively, i.e. /please check/i

The ping on my Windows box outputs for example:

Ping request could not find host badhost. Please check the name and tr
+y again.
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Some notes:

• also add use strict, please
• line 11: check/ certainly fails to parse
• check your OS documentation on you flavour of ping: you may be able to use the return code on e.g. Unix or Cygwin: system("ping -c3 $host"); warn if $?;
• why @a=`` and "@a"=~ // instead of the simpler $a=`ping -c3 host 2>&1` or similar?
  I use 2?&1 to include output on stderr instead of just stdout; this might be part of your problem or just me subscribing to an overly cautious cargo-cult :).
• finally there's also Net::Ping

Resulting in e.g. this example, which should also run on e.g. cygwin:

use strict;
use warnings;
my @hostnames = qw(www.leo.org dhbvg.xxx);
foreach (@hostnames){
   # my $a = `echo pinging $_ 1>&2 ; ping -c3 $_ 2>&1 | tee -a /dev/tt
+y`;
   my $a = `ping -c3 $_ 2>&1`;
   if ($a =~ /unreach|unknown host/i){
      print " For $_ : problems: $a.";
   }
}
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cu & HTH, Peter -- hints may be untested unless stated otherwise; use with caution & understanding.