use strict;
use PerlIO::gzip;

( @ARGV == 1 and -f $ARGV[0] and $ARGV[0] =~ /\.gz$/ )
    or die "Usage: $0  file.gz\n";

open( IN, "<:gzip", $ARGV[0] ) or die "open failed on $ARGV[0]: $!\n";
while (<IN>) {
    # $_ contains one line of uncompressed text data per iteration
    do_something($_);
}