I would like to process a lengthy (binary) string of length N as a stream, chopping off the front part as the parsing goes along. 
     my $n = unpack("v", $str);
     $str = substr($str, 1);

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Now I have noted that my application considerably slows down on longer strings, and it seems that is due to the following effect, indicating that this approach scales as N^2 (instead of the naively expected N). 
$ time perl -e '$a = "a" x 20_000; $a = substr($a, 1) while length($a)
+;'
user    0m0.350s

$ time perl -e '$a = "a" x 40_000; $a = substr($a, 1) while length($a)
+;'
user    0m0.791s

$ time perl -e '$a = "a" x 80_000; $a = substr($a, 1) while length($a)
+;'
user    0m2.683s

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A similar issue was discussed in Access via substr refs 2000 times slower: I think the reason for this slowdown is that when constructing support for the lvalue property of substr, a copy is made. Is there a good workaround to this scaling issue? Instead of passing an ever-shrinking stream around different functions, I could just pass the whole string and an offset, 
     my $n = unpack("v", substr($str, $off, 2));
     $off += 2;

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but this seems rather clumsy to me... Are there more elegant ways of doing this?

Update: Thanks for all the help and for the insightful explanations! As suggested, the 4-argument version of substr (setting REPLACEMENT to "") instead of the (2|3)-argument substr solves this in a very neat manner, 
    my $n = unpack("v", substr($str, $off, 2, ""));

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In reply to Using substr to split a string scales as N^2 -- alternatives? by hrr

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