As a quick-start, you could wrap the interactive part like so:

...
if (@ARGV) {             # test if there is at least one argument
  $basename = $ARGV[0];  # set basename to first argument
} else {                 # otherwise be interactive
  print "Enter base name of input files\n";
  chomp ($basename = <STDIN>);
}
...
[download]

Now, you can provide the filename as first argument or fall back into interactive mode. See e.g. Getopt::Long for another approach. Be careful with $var = shift; since it behaves different when called from inside or outside of a subroutine.