# untested

my $str='P=HierStehtIrgendwas,I=NocheinText';

# long way (with room for further checks (defined, valid values, ...)
my %data;
for my $pair ( split /,/, $str ) {
  my ( $key, $value ) = split /=/, $pair;

  $data{$key} = $value;
}

# if you are sure, that data in $str is always valid
my %data = split /[=,]/, $str;