my $Usage = "Usage:  $0  infile  outfile\n";

# open input and output files
die $Usage unless ( @ARGV == 2 );

open( IN, $ARGV[0] ) or die "Unable to read $ARGV[0]: $!\n$Usage";
open( OUT, ">$ARGV[1]" ) or die "Unable to write $ARGV[1]: $!\n$Usage";

...