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

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

...

converter.pl < some.input > some.output
# or
some_process | converter.pl | another_process
# or any combination of the above...

while (<IN>) {
    my @chars = split //;
...
        print $out;
    }
}