A quick test shows that substr is pretty intelligent about the way it operates. 

# OS reports memory use 3336k
my $s = ' ' x 1_000_000; 

# OS reports memory use 4320k

$s = substr $s, 0, 999_999; 

# OS continues to report 4320k

[download]

If substr was acting as a copy operator the memory would have to grow again to accomodate the copy. That is doesn't, even in this non-lvalue usage tends to indicate that the code has the smarts to recognise when the destination of a substr assignment is the same and the source and it performs a simple adjustment to the length of the SV in-situ, which is about a fast as is possible to be.

Examine what is said, not who speaks.
"Efficiency is intelligent laziness." -David Dunham
"Think for yourself!" - Abigail
Hooray!

In reply to Re: Efficienty truncating a long string by BrowserUk
in thread Efficienty truncating a long string by dino

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