It sounds like there are only a certain number of parameters you consider "valid"... You can explicitly specify them and just ignore the rest of the hash.

my @valid = qw/buttons action foo bar xyz/;
my %args = (
    buttons => [],
    action  => 'parse.cgi',
    @_
);

%args = map { exists $args{$_} ? $_ => $args{$_} : () }
        @valid;

while (my ($k,$v) = each %args) {
   ## etc...
}
[download]

This code is careful not to add additional key=>undef pairs to the hash, thus the exists check.

Alternately, you could just iterate over all valid keys and only print the ones that are available in the hash (instead of using each). This method would always print the list in a predictable order, if that's important to you:

my @valid = qw/buttons action foo bar xyz/;
my %args = (
    buttons => [],
    action  => 'parse.cgi',
    @_
);

foreach my $k (@valid) {
    next if not exists $args{$k};
    my $v = $args{$k};
    $v = @$v if ref $v eq 'ARRAY';

    print "$k => $v\n";
}
[download]

blokhead