# Discard every other line
open FIN, 'INFILE' or die;
open FOUT, '>INFILE.new' or die;
my $cnt;
while (<FIN>) {
   next if $cnt%2;
   print FOUT;
}
close FH;
close FOUT;
rename 'INFILE', 'INFILE.bak';
rename 'INFILE.new', 'INFILE;