The somewhat sucky way is:

my $string= ' ' x 2**19;
$string= '';
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It'd be cool if the following worked but it doesn't:

length( $string )= 2**19; # Doesn't work
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Or you can get creative:

open NUL, "<", File::Spec->devnull();
sysread( NUL, $string, 2**19-length($string), length($string) );
close NUL;
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Testing in my environment shows that you can even get away with:

sysread( STDOUT, $string, 2**19 );
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which leaves $string's current value unchanged while extending the storage allocated to it. Of course, there is a risk of running into a system where STDOUT is open for both read and write access. I wonder if there is another Perl built-in that reads into a buffer that can be used more conveniently.

Update: Testing also shows the following works well:

sub grow
{
    sysread( DATA, $_[0], $_[1] );
}
__DATA__
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