Hello PatGro, and welcome to the Monastery!

I'll read more on closure later today

• Start with the FAQ: What's a closure?.
• If you have access to the Camel Book¹, you can read about closures in Chapter 8: References.
• The Monastery has a tutorial: Closure on Closures.
• For in-depth study, Higher-Order Perl (2005) by Mark Jason Dominus is hard to beat. The opening chapters show how closures work-in naturally with other features of Perl to facilitate elegant solutions to common programming problems. The book is available free online at http://hop.perl.plover.com/.

¹Tom Christiansen, brian d foy & Larry Wall with Jon Orwant, Programming Perl, 4^th Edition, 2012.

Hope that helps,

Update: Added author links.