If this is ActiveState Perl built with VC6, I remember some bad (re)allocation behaviour of the runtime memory allocator. Maybe the decompressor reallocates the buffer quite often and triggers the bad behaviour.

A potential workaround could be to presize the buffer:

$cleartxt= " " x 100_000; # or whatever you expect;
$cleartxt= '';
gunzip ...
[download]

Super Search for a post by BrowserUk somewhere in such a thread could unearth more information.