"use warnings;" gives a clue as to the cause of this behaviour.

The problem is fixed by using
our $b = 'blue';
instead of "my".

"An our declaration declares a global variable that will be visible across its entire lexical scope, even across package boundaries. "

Offense, like beauty, is in the eye of the beholder, and a fantasy.
By guaranteeing freedom of expression, the First Amendment also guarantees offense.

In reply to Re: Passing Variables to External Subroutines (Perl/Tk) by NetWallah
in thread Passing Variables to External Subroutines (Perl/Tk) by antioch

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