#!/usr/bin/perl
# (use a unix/linux style shebang line,
# because someday you will want to use a unix/linux system)

use strict;

my $reqd_param_count = 2;  # (e.g. two file names)

if ( ! @ARGV ) {
    # prompt for interactive input of required parameter(s)
    ...
}
elsif ( @ARGV == $reqd_param_count ) {
    # invoked from an interactive shell: required params are in @ARGV
    ...
}
else {
    die "Usage:  $0  arg1 ...\n";
}