Pass them to your subs by reference.

sub fishes
{
    my $fishHoH = shift;
    for my $key (keys %{$fishHoH}) {
        print "$key fish: ", $fishHoH->{$key}->{fish}, "\n";
    }
}

my %HoH = ( one => { fish => 'red' },
            two => { fish => 'blue' } );

&fishes(\%HoH);
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-Bird