open(INPUT,"<$bigfile") ||die "Could not open $bigfile:$!\n";
open(OUTPUT,">$newfile")||die "Could not open $newfile:$!\n";
while (<INPUT>) {
  print OUTPUT $_      # note no semicolon here...
  unless /whatever condition matches the patterns you want/;
}
close(INPUT) || die "Could not close $bigfile: $!\n";
close(OUTPUT)|| die "Could not close $newfile: $!\n";
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