try this
$perm = $acl.Access | select IdentityReference, FileSystemRights | where {$_.IdentityReference -notin @("BUILTIN\Administrators", "NT AUTHORITY\SYSTEM")}
$perm | %{yourfunction $_.IdentityReference}
or on the same line
$acl.Access | select IdentityReference, FileSystemRights | where {$_.IdentityReference -notin @("BUILTIN\Administrators", "NT AUTHORITY\SYSTEM")} | %{yourfunction $_.IdentityReference}
solved Powershell apply a function to each member of a list