Group-S3ObjectKey.ps1

[CmdletBinding()]
param (
    [Parameter(Mandatory = $true)]
    [string]
    $BucketName,
    [Parameter()]
    [string]
    $Prefix,
    [Parameter(Mandatory = $true)]
    [ValidateRange("NonNegative")]
    [int]
    $Level
)
$params = @{
    "BucketName" = "$BucketName";
}
if ($Prefix -ne "") {
    $params["Prefix"] = $Prefix
}
Get-S3Object @params | ForEach-Object -Process { try { $_.Key.Split('/')[$Level] } catch {} } | Group-Object