It looks like your $sql_result is not defined and so $temp_hash becomes a scalar, which is not defined.

Certainly not. $temp_hash{undef} is entirely distinct from $temp_hash.

use strict;
my %t; my $t;
my $k;
$t{$k}='toto';
$t='titi';
# print 'toto', not 'titi'
print $t{$k}, "\n";
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