regexes carry with them a performance penalty.
there is nothing wront with just:

$foo = substr($foo,$n) unless(length $foo => $n);
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or

$substr($foo,0,$n)="" unless(length $foo => $n);
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or

$substr($foo,0,$n,'') unless(length $foo => $n);
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this is generally faster than a regex ;-)

Originally posted as a Categorized Answer.