The Questions

        perlfaq1: General Questions About Perl
        perlfaq2: Obtaining and Learning about Perl
        perlfaq3: Programming Tools
        perlfaq4: Data Manipulation
        perlfaq5: Files and Formats
        perlfaq6: Regular Expressions
        perlfaq7: General Perl Language Issues
        perlfaq8: System Interaction
        perlfaq9: Networking

##</code><code>##

Found in /System/Library/Perl/5.10.0/pods/perlfaq4.pod
       How can I find the Julian Day?

       (contributed by brian d foy and Dave Cross)

       You can use the "Time::JulianDay" module available on CPAN.  Ensure
       that you really want to find a Julian day, though, as many people have
       different ideas about Julian days.  See
       http://www.hermetic.ch/cal_stud/jdn.htm for instance.

       You can also try the "DateTime" module, which can convert a date/time
       to a Julian Day.

               $ perl &#8722;MDateTime &#8722;le'print DateTime&#8722;>today&#8722;>jd'
               2453401.5

       Or the modified Julian Day

               $ perl &#8722;MDateTime &#8722;le'print DateTime&#8722;>today&#8722;>mjd'
               53401

       Or even the day of the year (which is what some people think of as a
       Julian day)

               $ perl &#8722;MDateTime &#8722;le'print DateTime&#8722;>today&#8722;>doy'
               30
               

##</code><code>##

       map BLOCK LIST
       map EXPR,LIST
               Evaluates the BLOCK or EXPR for each element of LIST (locally
               setting $_ to each element) and returns the list value composed
               of the results of each such evaluation.  In scalar context,
               returns the total number of elements so generated.  Evaluates
               BLOCK or EXPR in list context, so each element of LIST may
               produce zero, one, or more elements in the returned value.

                   @chars = map(chr, @nums);

               translates a list of numbers to the corresponding characters.
               And

                   %hash = map { get_a_key_for($_) => $_ } @array;

               is just a funny way to write

                   %hash = ();
                   foreach (@array) {
                       $hash{get_a_key_for($_)} = $_;
                   }

               Note that $_ is an alias to the list value, so it can be used
               to modify the elements of the LIST.  While this is useful and
               supported, it can cause bizarre results if the elements of LIST
               are not variables.  Using a regular "foreach" loop for this
               purpose would be clearer in most cases.  See also "grep" for an
               array composed of those items of the original list for which
               the BLOCK or EXPR evaluates to true.

               If $_ is lexical in the scope where the "map" appears (because
               it has been declared with "my $_"), then, in addition to being
               locally aliased to the list elements, $_ keeps being lexical
               inside the block; that is, it can’t be seen from the outside,
               avoiding any potential side&#8208;effects.

               "{" starts both hash references and blocks, so "map { ..."
               could be either the start of map BLOCK LIST or map EXPR, LIST.
               Because perl doesn’t look ahead for the closing "}" it has to
               take a guess at which its dealing with based what it finds just
               after the "{". Usually it gets it right, but if it doesn’t it
               won’t realize something is wrong until it gets to the "}" and
               encounters the missing (or unexpected) comma. The syntax error
               will be reported close to the "}" but you’ll need to change
               something near the "{" such as using a unary "+" to give perl
               some help:

                   %hash = map {  "\L$_", 1  } @array  # perl guesses EXPR.  wrong
                   %hash = map { +"\L$_", 1  } @array  # perl guesses BLOCK. right
                   %hash = map { ("\L$_", 1) } @array  # this also works
                   %hash = map {  lc($_), 1  } @array  # as does this.
                   %hash = map +( lc($_), 1 ), @array  # this is EXPR and works!

##</code><code>##

use strict;
use warnings;
use diagnostics;