It's probably faster to use the C-style for loop,

for ( my $i=0; $i <= $max; $i++ ) {
}
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On the other hand, you might even want to do this whole loop from within C itself. See ch 21.3. Extending Perl (Using C from Perl) of Programming Perl by Larry Wall, Tom Christiansen, and Jon Orwant (ISBN 059600027).

erm correction, I mean do the substring extraction from C not the processing you do with it. Although the perl loop will indeed consume less resources if C-style instead of forcing a huge list to be constructed.