A good start would be to post the code you have, and examples of the data you want to provide to it. This helps us see much better where you are.

Ideally you also show the output you get from the code and also the output you expect.

An example structure could be:

#!perl
use 5.020;
use feature 'signatures';
no warnings 'experimental::signatures';

sub day_before( $day ) {
    # ...
}

for my $date (@ARGV) {
    say sprintf "The day before %s is %s", $date, day_before($day);
}
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