#!/usr/bin/perl -w       #added
use strict;              #added
use Data::Dumper         #added

my $host = $ARGV[0];        #update was 'hosta';
my $fsys = $ARGV[1];        #update was qq|/the/second/filesystem|;
my %filesystems = (
   hosta => {               #update
         desc => 'x', 
         filesystems => {   #update
                     qq|/the/first/filesystem| => 0,
                     qq|/the/second/filesystem| => 0
         }                  #update
   },                       #update
   hostb => {               #update
         desc => 'r', 
         filesystems => {   #update
                     qq|/the/first/filesystem| => 0,
                     qq|no filesystemB for 2nd host | => 0
         }                  #update
   }                        #update
);

print Dumper(\%filesystems); #added

if ( ! exists $filesystems{$host}{filesystems}{$fsys} ) {
        print "error\n";
}
else {
        print "Found filesystem $fsys on host $host\n"
        . $filesystems{$host}{descr} . "\n";
}