Not my work, but a great example:

sub IsLeapYear
{
   my $year = shift;
   return 0 if $year % 4;
   return 1 if $year % 100;
   return 0 if $year % 400;
   return 1;
}
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The IsLeapYear subroutine is called with a year number, like

IsLeapYear(2006);
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The function returns a true value if the year number is a leap year, false otherwise.

Get the correct number of days in February for the given year:

my $days_in_February = IsLeapYear($year) ? 29 : 28;
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