Since you understand the shift and logical or ||, then maybe a simple example will illustrate the usage:

use warnings;
use strict;

my $age = 15;

print get_age($age); #  my age is: 15
print "\n";
print get_age();     #  my age is: 200

sub get_age {
    my $my_age = shift || 200;
    return " my age is: ", $my_age;
}
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