~/$ perl -e ' 
$string="sometrash key1=value0 value1, value2 key2=value3 key3=value4"; 

%hash = $string =~/
(\w+)            #capture the key name
=                # separated from the value by an equals
(.+?)            # and then the value, non-greediness prevents running into the subsequent values
(?=(?:\s\w+=|$)) # finally we look ahead to ensure that what follows is a space and a "key=" pattern, or else the end end of the string
/xg;             #Match globally and allow comments in the regex for maintainability

# And check
use Data::Dumper;print Dumper(\%hash);
'
$VAR1 = {
          'key2' => 'value3',
          'key1' => 'value0 value1, value2',
          'key3' => 'value4'
        };