frobnitz( $HoA{Foo} )   if  $HoA{Foo};
for my $key (  keys %HoA  ) {
    frobnitz( $HoA{$key} )   if  $key ne "Foo";
}

##</code><code>##

my %HoA= (
    D=>[4], E=>[5], F=>[6], Foo=>[3], G=>[7], H=>[8],
);
{
    local( $HoA{Foo} )= $HoA{Foo};
    for my $av (  delete $HoA{Foo}, values %HoA  ) {
        print $av->[0], $/   if  $av;
    }
}
print $HoA{Foo}[0], $/;

##</code><code>##

sub FirstIfKeys {
    my $hv= pop @_;
    my %k;
    @k{ keys %$hv }= (1) x @k;
    my @k;
    for my $key (  @_  ) {
        push @k, $key   if  delete $k{$key};
    }
    return @k, keys %k;
}

    for my $key (  FirstIfKeys( "Foo", \%HoA )  ) {
        ...
    }