#1 No explanation of how to specify an input file

[Ari Friedman]

There is no explanation in gdec.pl of how to specify an input file except inside the source. Perhaps on line 14:
open(IN,$input) or die "Can't find input file!";

you could add it, like this:
open(IN,$input) or die "Can't find input file! Usage: gdec.pl -i <inputfile>";

G'luck,
Ari