Strange difference between $_ and [parameter(ValueFromPipeline=$true)]

I've come to the edge of my PoweShell-fu. Can someone explain to me why these two functions act differently when piping an array of arrays? All that's different is whether I'm using `$_` or `[parameter(ValueFromPipeline=$true)] $input` to get at the piped input. I expected those to act identically in this situation. ``` $pairs = ('a', 'b'), ('c', 'd') function dollarUnderscoreFunction { Process { Write-Host "`$_[0] = $($_[0])" Write-Host "`$_[1] = $($_[1])" } } function pipedParameterFunction([parameter(ValueFromPipeline=$true)] $input) { Process { Write-Host "`$input[0] = $($input[0])" Write-Host "`$input[1] = $($input[1])" } } Write-Host "`$pairs:" $pairs | foreach { Write-Host $_ } Write-Host "`nRunning dollarUnderscoreFunction`n" $pairs | dollarUnderscoreFunction Write-Host "`nRunning pipedParameterFunction`n" $pairs | pipedParameterFunction ``` Output in PowerShell v3: ``` $pairs: a b c d Running dollarUnderscoreFunction $_[0] = a $_[1] = b $_[0] = c $_[1] = d Running pipedParameterFunction $input[0] = a b $input[1] = $input[0] = c d $input[1] = ``` Output in PowerShell v2: ``` $pairs: a b c d Running dollarUnderscoreFunction $_[0] = a $_[1] = b $_[0] = c $_[1] = d Running pipedParameterFunction [ : Unable to index into an object of type System.Collections.ArrayList+ArrayListEnumeratorSimple. At C:\Untitled1.ps1:16 char:8 + $input[ <<<< 0] + CategoryInfo : InvalidOperation: (0:Int32) [], RuntimeException + FullyQualifiedErrorId : CannotIndex $input[0] = [ : Unable to index into an object of type System.Collections.ArrayList+ArrayListEnumeratorSimple. At C:\Untitled1.ps1:17 char:8 + $input[ <<<< 1] + CategoryInfo : InvalidOperation: (1:Int32) [], RuntimeException + FullyQualifiedErrorId : CannotIndex $input[1] = [ : Unable to index into an object of type System.Collections.ArrayList+ArrayListEnumeratorSimple. At C:\Untitled1.ps1:16 char:8 + $input[ <<<< 0] + CategoryInfo : InvalidOperation: (0:Int32) [], RuntimeException + FullyQualifiedErrorId : CannotIndex $input[0] = [ : Unable to index into an object of type System.Collections.ArrayList+ArrayListEnumeratorSimple. At C:\Untitled1.ps1:17 char:8 + $input[ <<<< 1] + CategoryInfo : InvalidOperation: (1:Int32) [], RuntimeException + FullyQualifiedErrorId : CannotIndex $input[1] = ```