See "User-defined subroutines" section in Learn Perl in about 2 hours 30 minutes

See "The Default Array Variables" and Chapter 5 "Functions" in the free book Modern Perl a loose description of how experienced and effective Perl 5 programmers work....You can learn this too.

#!/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;
}
[download]

Tutorials: Variable Scoping in Perl: the basics,
Coping with Scoping , Mini-Tutorial: Perl's Memory Management,
Lexical scoping like a fox,
Read this if you want to cut your development time in half!,
Closure on Closures , perlref#Circular References,
Memory leaks and circular references,
Circular references and Garbage collection.,
make perl release memory,
about memory management , Re^3: Scope of lexical variables in the main script , Re: What's the weaken here for?,
Devel::Cycle,
Devel::NYTProf,
Devel::Leak Devel::LeakTrace,
WeakRef

see Re: No such file or directory error/No such file or directory error, see template at (tye)Re: Stupid question (and see one discussion of that template at Re^2: RFC: Creating unicursal stars