$arr[ ($i - 4) .. $i ]


##</code><code>##

my $h = $i - 4;
if ( $h >= 0 ) {
    if (unequal ( $arr[ $h .. $i ] ) {
         # flag stuck value
    }
}

# ...

sub unequal {
    my %v = map { ($_, 1 ); } @_;
    my @v = keys %v;
    return $#v;
}