if (!defined ($query->param('loc')))
{
     $user->thwap();
}
[download]

We should also check whether it has a length in this case, since it could be the empty string (e.g., if loc=&foo=bar is passed).

a null '' is actually undef, it equals to false, so

unless ($query->param('loc')) {
    # print error mesage or whatever.
}
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if ($query->param('loc) eq '') {
    ...
}
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does not work...

you might want to take a look at What is truth? here at the monastery for an explanation of "truth" in perl, as there are many 'gotchas' in handling truth in perl ;) (for example, in the code above, if 0 was a valid value for the loc parameter, the code would print the error message, even if we didn't want that.)

hope this helps,

As was pointed out, it might be better to check using length instead of defined. Remember that zero is false, so a straight logic check will not tell you if it is supplied but zero. Consider:

unless ($query->param('is_a_chicken')) # Fails on 0
{
     print "You must choose an option.<BR>\n";
}
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As your appropriate values might be 0 or 1, this is going to fail on a valid submission. Better to have:

unless (length $query->param('is_a_chicken'))
{
     print "You must choose an option.<BR>\n";
}
[download]

Though length does tend to complain about being forced to deal with undefined strings. This will lead to the more verbose but equally more robust version:

my $is_a_chicken = $query->param('is_a_chicken');
unless (defined ($is_a_chicken) && length ($is_a_chicken))
{
     print "You must choose an option.<BR>\n";
}
[download]

Of course, if you were doing this sort of thing all the time you could make a quick helper method to do it quickly:

sub provided
{
     return defined($_[0]) && length ($_[0]);
}
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Which would simplify as follows:

unless (provided ($query->param('is_a_chicken'))
{
     print "You must choose an option.<BR>\n";
}
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if(!$query->param){
     do_stuff();
}elsif($query->param('loc')){
     do_something_with($query->param('loc'))
}else{
     error_page(); #for invalid parameters
}
[download]