# Old-style: Bareword filehandles and two-arg opens:
open IN,  "zcat $in_filename|"         or die;
open OUT, "|gzip -c - > $out_filename" or die;

# New style: Lexical filehandles and three-arg opens
open my $in_fh,  '-|', "zcat $in_filename"         or die;
open my $out_fh, '|-', "gzip -c - > $out_filename" or die;