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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!