The syntax in the if statement is that of a hash of hashes, four deep. Below is what I think you are really trying to do.

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

my %hash_data = ();

#my $key1 = "a";
#my $key2 = "1";
#my $key3 = "B";
#my $key4 = "_";

my @keys = ("a", "1", "B", "_");
$hash_data{a} = "aValue";
for my $key (@keys) {
   if (exists $hash_data{$key}) {
      print "exists\n";
   } else {
      print "DOES NOT EXIST\n"
   }
}
print "-----\n";
print Dumper (\%hash_data);
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