Unfortunately, with the look-ahead you would miss the last entry in your example

Just put an alternation in the look-ahead to cope with end of string.

knoppix@Microknoppix:~$ perl -MData::Dumper -Mstrict -wE '
> my $str =
>    q{sometrash key1=value0 value1, value2 key2=value3 key3=value4};
> my %data = $str =~ m{([^\s=]+)=(.*?)(?=(?:\s+[^\s=]+=|\z))}g;
> print Data::Dumper->Dumpxs( [ \ %data ], [ qw{ *data } ] );'
%data = (
          'key2' => 'value3',
          'key1' => 'value0 value1, value2',
          'key3' => 'value4'
        );
knoppix@Microknoppix:~$
[download]

I hope this is of interest.