Here you go:

    Function Do-WhatOPAskedFor
    {
	    param
	    (
		    [Microsoft.SharePoint.SPList] $list
    	)

        # Please note that this is a bad way of getting all items from the list!
	    $items = $list.Items
    	$items | Align-DateFields "Modified" "Date Last Saved"
    }

    Function Align-DateFields
    {
	    param
    	(		
	    	[string] $dateModifiedFieldName,
    		[string] $dateLastSavedFieldName,
	    	[Parameter(Mandatory=$true,ValueFromPipeline=$true,ValueFromPipelineByPropertyName=$true)]
	    	[Microsoft.SharePoint.SPListItem] $item
    	)
	
    	begin {}
	    process
    	{
	    	[DateTime]$dateLastSaved = $item[$dateLastSavedFieldName];
		    $item[$dateModifiedFieldName] = $dateLastSaved;
    		$item.Update();
    	}
    	end {}
    }

`Do-WhatOPAskedFor` will do what you asked for; I'm just bad at naming functions.  What it does is takes a SharePoint list, enumerates its items, and sends each item to be processed by a function.  The function `Align-DateFields` takes a SharePoint list item, extracts what's in the specified field, and saves it to another field.

Let me know if you need any further details!

EDIT:

Here's some code to load up the SharePoint list and pass it to the function above:

    # Do this if you can load the SharePoint module (e.g., are in a SharePoint Management Shell)
    $web = Get-SPWeb http://yourwebsitehere;
    $list = $web.Lists["Your list title here"];
    Do-WhatOPAskedFor $list;
    $web.Dispose();

    # Do this if you only have a regular PowerShell shell:
    [System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint") | Out-Null;
    $site = New-Object Microsoft.SharePoint.SPSite("http://yoursitecollectionhere");
    $web = $site.AllWebs | Where { $_.Title -eq "Your site title here" } | Select -First 1
    $list = $web.Lists["Your list title here"];
    Do-WhatOPAskedFor $list;
    $site.Dispose();

In the first one, we use the functions in the SharePoint PowerShell module to grab the `SPWeb`, extract the `SPList` and pass it to the function.  In the second, we have to load the `Microsoft.SharePoint` assembly, instantiate an `SPSite` object which contains the `SPWeb`, grab the `SPWeb`, extract the `SPList`, and pass it to the function.