use strict;   # DON'T FORGET TO
use warnings; # DO THESE!

# the shift is implicitly acting on @ARGV.
my $base_name = shift;
unless ( $base_name )
{
    print "Enter base name of input files: ";
    chomp ( $base_name = <STDIN> );
}

# die() or retry a read from <STDIN> if there
# is no $base_name at this point.

# Then check to see if $base_name has a valid value.

# Continue on. Happy Perl!
[download]