It's creating an anonymous array, just like 
%hash = map {$_ => undef } qw(city state country);

[download]
which produces a list like
( 'city' => undef, 'state' => undef, 'country' => undef )

[download]
which is then turned into a hash. In fact, any other value than undef would do just as well, as it isn't actually used.

But, it does so for every loop, just to test if $field is one of qw(city state country). That's a very inefficient way of doing it.

A better way would be to create the hash outside of the loop: 

my %skip = map {$_ => undef } qw(city state country);

foreach my $field (param) {
  next if exists $skip{$field};
  foreach my $value (param($field)) {
     #process non blank thedata variable
     ...
  }
}

[download]

Or, IMO perfectly acceptable for small skip lists and/or few parameters: 

foreach my $field (param) {
  next if grep { $field eq $_ } qw(city state country);
  foreach my $value (param($field)) {
     #process non blank thedata variable
     ...
  }
}

[download]

Note that I moved the test outside of the inner loop, as the result won't change for whatever values you get.

In reply to Re: Explanation of mapping and undef by bart
in thread Explanation of mapping and undef by Anonymous Monk

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.