Here is non-regexp solution. It validates input in addition to the main task.

sub splitlong {
    my $text = shift;
    my @result;
    my ($cnt, $inside) = (0, 0);
    for (split //, $text) {
        ($inside = 0), next if $inside and $_ eq ']';
        $inside = 1 if !$inside and $_ eq '[';
        next if $inside;
        $cnt = /\s/s ? 0 : $cnt + 1;
        ($cnt = 0), push @result, '-' if $cnt > 3;
    }
    continue {
        push @result, $_;
    }
    warn "invalid input: $text" if $inside;
    return join '', @result;
}

my $text = 'ju[color=1]s[/color]t a sam[color=2]p[/color]le';

print splitlong($text);
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