What's going on in your initial code is when a duplicate key is encountered, you attempt to deference a string -- in the case of key one, you are accessing the non-existent variable @1. Your value then gets pushed onto @1, and not the hash element. The appropriate action is to test the values of the hash to see if they exist, and act accordingly, a la:

#!/usr/bin/perl -w

use strict;
use Data::Dumper;

my %h1 = (    "one"   => 1,
              "two"   => 2,
              "three" => 3,   );  


my %h2 = (  "four"  => 4,
            "five"  => 5,
            "six"   => 6,
            "one"   => 11111, );  

foreach my $x ( keys %h2 ){
    if (exists $h1{$x}) {
        $h1{$x} = [$h1{$x}, $h2{$x}]
    } else {
        $h1{$x} = $h2{$x};
    }
}
print Dumper (\%h1);
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In general, I find accessing data structures with inconsistent structure irritating and way too bug prone. Rather that varying structure based upon whether there is one or more value, I would make every hash entry an array reference, a la:

#!/usr/bin/perl -w

use strict;
use Data::Dumper;

my %h1 = (    "one"   => 1,
              "two"   => 2,
              "three" => 3,   );  


my %h2 = (  "four"  => 4,
            "five"  => 5,
            "six"   => 6,
            "one"   => 11111, );  

foreach my $x ( keys %h1 ){
    $h1{$x} = [$h1{$x}];
}
foreach my $x ( keys %h2 ){
    push @{$h1{$x}}, $h2{$x};
}
print Dumper (\%h1);
[download]