Fore! 38 strokes... 
#23456789_123456789_123456789_12345678
$_=pop;for$x(1..pop){y/a-z/b-za/}print

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UPDATE: Looking at other solutions inspired an improvement to 36 strokes.
#23456789_123456789_123456789_123456
$_=pop;eval"y/a-z/b-za/;"x pop;print

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In reply to Re: Golf: ROT-n by tilly
in thread Golf: ROT-n by hardburn

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