I personally believe that for the benefit of the OP it would be sensible to explain that while this does exactly what he asked for: namely "select" non zero values within the C<for> clause, it would be "inefficient" (in some sense) in that grep is a loop in disguise. Thus with such code one is iterating twice over... well, not just the same list, but two strictly correlated ones. Literally, it's like one had been doing:

my @nonzero;

for (keys %is_available) {
    push @nonzero, $_ if $is_available{$_};
}

for my $p9 ( @nonzero ) {
    print $p9;
}
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