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#!/usr/bin/perl -w

...

$ENV{'abc'} = 'def';
system('echo $abc');

% echo $abc
abc: Undefined variable.

  [Parent process] ======> [Perl subprocess] ====> [Sub process shell]
    Env list #1,             Env list #2              Env list #3
   "abc" NOT defined        copied from the          copied from the
                            Parent process,         Perl subprocesses,
                           define "abc" here       "abc" still defined