sub is_fibonnacci {
    my $plus = (5 * $_[0] * $_[0]) + 4;
    my $mins = (5 * $_[0] * $_[0]) - 4;
    return is_perfect_square($plus) | is_perfect_square($mins);
}

##</code><code>##

sub is_fibonnacci {
    my $plus = (5 * $_[0] * $_[0]) + 4;
    my $mins = $plus - 8;
    return is_perfect_square($plus) || is_perfect_square($mins);
}

##</code><code>##

sub is_perfect_square {
    my $sqrt = int(sqrt($_[0]));
    return $sqrt * $sqrt == $_[0];
}

##</code><code>##

sub is_perfect_square { int( $_[0] ** .5 ) ** 2 == $_[0] }