Another solution without using any module: 
#!/usr/bin/perl -w
use strict;

while (1) {
    print "Enter year (yyyy) and month (mm), separated by a space:";
    chomp(my $input = <STDIN>);
    last if !$input;

    my ($year, $month) = split / /, $input;
    my $m= $month % 12 + 1;
    my $y= $year+int($month/12);
    if ( $m<3) {
        $m+=12;
        --$y;
    }
    ++$m;
    my $dayname = (qw(Sat Sun Mon Tue Wed Thu Fri))[(int($m * 30.6) + 
+int($y * 365.25)-int($y / 100)+int($y / 400)) % 7];

    print "The last day of $month/$year will be a $dayname\n";
}

[download]
The formular used here works like this:
  1. if the month is January or february (1 or 2) shift it to the end of the preceeding year (month 13 and 14)
  2. add 1 to the month
  3. sum up the integer result of:
    1. month * 30.6
      30.6 gives it the correct toggle between 30 and 31. February is the last month, so no problem with 28/29
    2. year * 365.25
      this takes into account the leap years
    3. year / -100
      this subtracts the non-leap years
    4. year / 400
      this re-adds the non-non-leap years
  4. the day should then be added
the reminder of 7 is the weekday starting with Sat==0
s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

In reply to Re: Determining the dayname for the last day of any given month by Skeeve
in thread Determining the dayname for the last day of any given month by McDarren

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