There is no way that I know of to tell Perl that assigning a different value to a hash key is an error. That is because that is defined in the language to be completely ok!

If you want checking like that, then the way to do it is to read the data like it came from an external file, build the hash and report errors as the data is processed.

There are many formulations of how to do this. I present one below. 

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

my $hash_data = <<END; #this is a "here is" string
one 1
two 2
one 3
END

my %seen;
my %hash = 
    map{ 
          my ($text, $number) = split;
          warn "\"$text\" has been re-defined!\n" if $seen{$text}++;
          ($text,$number);
       }split (/\n/, $hash_data);

print Dumper \%hash;

__END__
Prints:
"one" has been re-defined!
$VAR1 = {
          'one' => '3',
          'two' => '2'
        };

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In reply to Re: Throw compilation error(or warning) if duplicate keys are present in hash by Marshall
in thread Throw compilation error(or warning) if duplicate keys are present in hash by I_love_perl

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