substr is so much simpler here.

while( $sz > 32760 ) {
   print $FH substr($m, pos($m), 32760);
   pos($m) += 32760;
   $sz -= 32760;
}

print $FH substr($m, pos($m), $sz);
pos($m) += $sz;
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Or if you don't mind destroying $m,

while( $sz > 32760 ) {
   print $FH substr($m, 0, 32760, '');
   $sz -= 32760;
}

print $FH substr($m, 0, $sz, '');
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But I fail to see how those are any different than just

print $FH substr($m, pos($m), $sz);
pos($m) += $sz;
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and

print $FH substr($m, 0, $sz, '');
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