Though we have arrived at very similar solutions, I think yours fails to pick up multiple values for the last key:

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

my $data="key1=value1 key2=value2 value3 key3=value4 value5 value6 key
+4=value7 key5=value8 key6=value9 value10";

# Albannach version
my %hash;
while ($data =~ /(\w+=)((\w+( |$)+)+)/g) {
  $hash{$1} = $2;
}
print Dumper \%hash;

# gumpu version:
my %foo;
while ($data =~ m/(\S+)=(\S+($|([^=]+\s+)+))/g) {
  $foo{$1}=$2;
}
print Dumper \%foo;
[download]

Produces:

$VAR1 = {
          'key4=' => 'value7 ',
          'key5=' => 'value8 ',
          'key1=' => 'value1 ',
          'key6=' => 'value9 value10',
          'key2=' => 'value2 value3 ',
          'key3=' => 'value4 value5 value6 '
        };
$VAR1 = {
          'key1' => 'value1 ',
          'key2' => 'value2 value3 ',
          'key3' => 'value4 value5 value6 ',
          'key4' => 'value7 ',
          'key5' => 'value8 ',
          'key6' => 'value9 '
        };
[download]

Update: The = in the key? You are right of course - silly me! Thanks gumpu!

--
I'd like to be able to assign to an luser