Use the string increment operator:

$newfile = "0001";  # Initialize to 0001
++$newfile;         # Now contains 0002
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Same thing, but longer:

$newfile = "0001";                       # Initialize to 0001
$newfile = sprintf("%04d", $newfile+1);  # Now contains 0002
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If you just want 2 as opposed to 0002, then it's just:

$newfile = "0001";  # Initialzie to 0001
$newfile += 1;      # Now contains 2
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