#!/usr/bin/perl --
use strict;
use warnings;
...
Main( @ARGV );
exit( 0 );
sub Main {
   my $file = shift || 'thefyle.xxx' ;
   my $workbook = create_workbook( $file );
   ...
}
sub create_workbook {
    my( $filename ) = @_;
    my $workbook = ...;
    ...
    return $workbook;
}