Is there a way to introduce a variable which is defined in the main program and is seen from external modules.

Sure. See our.

What is a drawback of such a variable

There is extensive literature out there, for example search for why are global variables bad. In short: global state makes programs harder to understand, to debug, can cause problems with recursion and concurrency, and is often a sign of bad encapsulation.

Perl 6 - links to (nearly) everything that is Perl 6.

In reply to Re: globally seen variable by moritz
in thread globally seen vairable by vit

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