Here's another way.

# Args are N, Oldfile, Newfile
# Get N and Newfile, leave Oldfile
my ($n, $newfile) = (shift,pop);
# Process inplace
$^I=1;
open(NEW, '>', $newfile) or die "$!: $newfile\n";
while (<>) {
  # Print first N lines to NEW
  if (1..$.==$n) { print NEW }
  # Everything else overwrites Oldfile
  else { print }
}
close NEW;
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