Don't use global variables. Global variables are a symptom of a wider problem: global state.

Instead of relying on modules/subs picking up data from some sort of global place, you should pass the data to the function as a parameter.

# this is bad...
#
sub do_something {
   if ( $MyApp::VERSION < 1.0 ) {  # a global
      do_something_simple();
   }
   else {
      do_something_complex();
   }
}

do_something();

# this is good...
#
sub do_something {
   my ($version) = @_;
   if ( $version < 1.0 ) {  # a local variable
      do_something_simple();
   }
   else {
      do_something_complex();
   }
}

do_something(1.2);
[download]

Eventually you may find yourself passing around quite a few bits of data like this (a version number, maybe a database handle too, and a filename for writing output):

use v5.14;
use warnings;
use DBI ();

package MyApp {
   
   sub do_something {
      my ($version, $dbh, $output_file) = @_;
      if ( $version < 1.0 ) {  # a local variable
         do_something_simple($output_file);
      }
      else {
         do_something_complex($output_file, $dbh);
      }
   }
   
   sub do_something_simple {
      my ($output_file) = @_;
      open my $fh, '>', $output_file or die;
      print $fh "Hello world\n";
   }
   
   sub do_something_complex {
      my ($output_file, $dbh) = @_;
      open my $fh, '>', $output_file or die;
      print $fh "Hello world\n";
      printf $fh "There are %d rows in the table.\n",
         $dbh->do("SELECT 1 FROM my_table");
   }
}

MyApp::do_something(
   1.2,
   DBI->connect("dbi:SQLite:dbname=myapp.sqlite"),
   '/tmp/output.txt',
);
[download]

It can become cumbersome to pass them around from one function call to the next, and tricky to keep track of which functions need access to which data. That's where object-oriented programming becomes handy:

use v5.14;
use warnings;
use DBI ();

package MyApp {
   use Class::Tiny qw( version dbh output_file );
   
   sub do_something {
      my $self = shift;
      if ( $self->version < 1.0 ) {  # an object attribute
         $self->do_something_simple;
      }
      else {
         $self->do_something_complex;
      }
   }
   
   sub do_something_simple {
      my $self = shift;
      open my $fh, '>', $self->output_file or die;
      print $fh "Hello world\n";
      close $fh;
   }
   
   sub do_something_complex {
      my $self = shift;
      open my $fh, '>', $self->output_file or die;
      print $fh "Hello world\n";
      printf $fh "There are %d rows in the table.\n",
         $self->dbh->do("SELECT 1 FROM my_table");
      close $fh;
   }
}

my $app = MyApp->new(
   version      => 1.2,
   dbh          => DBI->connect("dbi:SQLite:dbname=myapp.sqlite"),
   output_file  => '/tmp/output.txt',
);
$app->do_something;
[download]

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