To your first question: we can't be sure the number of days is the SECOND CLI argument (which is what you're extracting in what you've shown -- $ARGV[1]); if the first CLI argument is what you want to feed to $days, then use $ARGV[0].

Here are two examples without the overhead of module loading:

#!/usr/bin/perl
use 5.018;
use warnings;

# 1110785

my $days;

if (@ARGV) {
    $days = $ARGV[0];
    say $days;
}else{
    say "n\t Usage: 1110785.pl n \n";
}

deleteData($days);

sub deleteData {
    $days = shift (@_);
    say "days in sub: $days";
}

say "back in main.";

=head EXECUTION

C:\>D:\_Perl_\PMonks\1110785.pl 3
3
days in sub: 3
back in main.

=cut
[download]

or (some would say better style):

 # ...
# 1110785a

my $days;

sub deleteData {

if (@ARGV) {
    $days = $ARGV[0];
    say $days;
}else{
    say "n\t Usage: 1110785.pl n \n";
}

$days = shift (@ARGV);
say "days in sub: $days";
#   ...
}

deleteData;
say "back in main.";

=head EXECUTION

C:\>D:\_Perl_\PMonks\1110785a.pl 8
8
days in sub: 8
back in main.

=cut
[download]

As to the your second question, why not 'try it to see?'