You know you can use substr as an lvalue or with a fourth parameter to avoid duplicating your big scalars.

If you need to use unpack to decode small chunks of the buffer, or pack to overwrite small chunks, use them in conjunction with substr to avoid copying:

my @decoded = unpack '...', substr $bigscalar, $offset, $size;

substr $bigscalar, $offset, $size, pack '...', @newValues;

# or
substr( $bigscalar, $offset, $size ) = pack '...', @newValues;
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But be very sure that the size specified in substr, and the size of the result from pack match exactly, otherwise you will be expanding or shrinking your big scalar by the difference which will lead to nasty surprises. It may be better to use an intermediary variable here:

my $replacement = pack '...', @newValues;

substr( $bigScalar, $offset, length $replacement ) = $replacement;
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It is also possible (from 5.8.5 onwards) to set up an array of lvalue references to chunks of your scalar and then manipulate the individual chunks through indirection:

## Create a scalar
perl> $bigScalar = 'the quick brown fox jumps over the laxy dog';;

## Create an array of lvalue refs to the indivdual words using \substr
+...
perl> @lvrefs = map{ 
    \substr $bigScalar, $_->[0], $_->[1] 
} [0,3], [4,5], [10,5], [16,3], [20,5], [26,4], [31,3], [35,4], [40,3]
+;;

## Indirecting through the elements of the array gives you the words
perl> print $$_ for @lvrefs;;
the
quick
brown
fox
jumps
over
the
laxy
dog

## And assign through the elements allows you to replace them, in-plac
+e, individually
perl> ${ $lvrefs[7] } = 'lazy';; ## The ${ ... } is necessary.

## The typo is now corrected.
perl> print $bigScalar;;
the quick brown fox jumps over the lazy dog
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