my %RULES = (
    'post' => { 'msgid' => '^(\d+|new)$', 'data' => '...', },
    'edit' => { 'msgid' => '^(\d+|new)$', },
    ...,
};

my $allowed = $RULES{ $query->{'action'} };

foreach $key (keys %$query) {
    exists $allowed->{'key'} ? 1 : return 0;
    ...
}


##</code><code>##

for ($query->{'action'}) {
    $allowed = /post/    ? { ... } :
               /edit/    ? { ... } :
               /delete/  ? { ... } :
                           die "that action isn't supported";
}


##</code><code>##

local $_ = "0A72656B636148206C72655020726568746F6E41207473754A";
while(s/..$//) { print chr(hex($&)) }