my $lim = 32700;
$s=~s/^ {1,$lim}//mg while ($s=~/^ /);
[download]

Let the RE engine do the work. (You can make this simpiler if you don't worry about lines with more then 32k blanks at the start: $s=~s/^ *//mg;.)

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Warning: Unless otherwise stated, code is untested. Do not use without understanding. Code is posted in the hopes it is useful, but without warranty. All copyrights are relinquished into the public domain unless otherwise stated. I am not an angel. I am capable of error, and err on a fairly regular basis. If I made a mistake, please let me know (such as by replying to this node).