Incremental strategy

• create your test suite first
• do little steps, and always test the result
• use a revision control system like git
• commit every change
• use branches for experiments
• once you found a previously untested bug expand your test suite and roll back

Prerequisites

Study and understand

==== Perl essentials

• strict and warnings
• my vs our variables,
• namespaces, scoping, blocks
• exporter
• constants
• warn and die

==== Tools and techniques

• the debugger
• trace options to log function calls
• log files
• Data::Dumper , Data::Dump

==== your application

• your data model ( database, configs)
• input and output
• avaliable doc
• stakeholders (users, contributers, maintainers) to ask

Analyze

• analyze the dependencies of your subs (x calls y calls z)
• analyze the shared variables
• try to visualize the dependencies in a graph
• the hierarchy should help identifying logical units (aka modules)
• use tools to help you analyze like B::Xref
• other tools like mentioned here: Searching for duplication in legacy code
• identify dead code (never called subs, out-commented trash)

Documentation

• once you understood a mechanism, write it down
• use Pod headers when possible
• normalize (beautify) your code with Perl::Tidy
• review your pod2html from time to time and fill gaps

Modularisation / Namespaces ?

• bundle subs into modules by functionality not technology (not all sql in one module, eg look at the TABLE names )
• require into the same namespace might be an easier intermediate step before learning to use exporter °
• modules normally require namespaces (packages)
• package variables in other modules need to be fully qualified when used outside $Pkg::var °
• same for Pkg::subs() °
• modules allow to export vars and subs when use d
• modules allow to pass and init shared variables when using import (like a database handle)

Object Oriented Programming

• logical modules are sometimes better OO classes
• check guides on "when OOP is better"
• indicator: if you have to pass around same arguments
• indicator: group of subs access always same global vars
• indicator: init() routines for globals are sometimes better ->new()
• easier to construct an object with encapsulated instance vars and class vars
• have a look at Moo before doing old style OOP

Improvement

• make your code more fault tolerant
• add argument checking to your subs
• rewrite many positional args into named args
• condense duplicated code into new functions
• limit the scope of vars and subs if possible
• give identifiers like variables meaningful names
• document your strategy for future maintainers

See Also

• Perl split file into modules

update

Expanded the OOP part

°) that's an intermediate step exporting and importing is much cleaner

²) As a rule of thumb from easier to better

• require files same namespace (NB: my-lexicals can't be shared)
• try to group connected subs into namespaces with package
• use module different namespaces => full qualified identifiers for shared data
• use module different namespaces => exporting and importing shared data
• use oo-class : shared data in instance/class-vars and methods, transport via constructor and setter/getters