Here is another explanation, or rather, a visual of what is going wrong with your code above --

#!perl -w

use strict;
use warnings;
use Data::Dumper;

my $array_ref = [1, 2, 5, 7, 4];

my %hash;
$hash{$array_ref} = 'ashok';

print Dumper($array_ref);
print Dumper(\%hash);

------ output --------
$VAR1 = [
          1,
          2,
          5,
          7,
          4
        ];
$VAR1 = {
          'ARRAY(0x2253b8)' => 'ashok'
        };
[download]

As you can see, in your hash, you have unintentionally tried to create a key using an array... as others have said, a key can only be a scalar. Whenever in doubt, try to use Data::Dumper to dump out what Perl is encountering. It is like debugging, without the debugger (wishes debugger were easier).

Good luck.