Define $basepath and $filename outside of the loop.

my ($basepath, $filename, $first_written);
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Inside the loop use something like:

$basepath = 'somewhere';
$filename=localtime;
do {&write($firstline, $basepath, $filename), $first_written++} unless
+ $first_written;
.
.
.
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And after the loop $basepath and $filename will still have their values, so you can just call &write($lastline, $basepath, $filename)