In your case, $_ is an alias (think pronoun) for the current list value being iterated in the closest loop.

I think the main problem you are having is the implicit loop that grep creates. grep actually creates another loop which sets $_.

Also your outside loop over @required never sets $_ since you explicitly create a variable.

ex.

my @required = qw(preserver sunscreen water_bottle jacket);

foreach my $item (@required) {
    print "$_\n"; ## You just get a warning
}
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