#!/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'
        };