That sentence in perlvar does not mean "changing %ENV only changes the environment of all children", it's more like "note that this will change the envirnonment of all your children".

Relax and test it, %ENV represents the current environmental variables of your process, without caveats.

#!/usr/bin/perl

$ENV{FOO} = 1;

print "\$ENV{FOO} is $ENV{FOO} still\n";

exec( '/usr/bin/env' );
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The output of env will also mention "FOO" being "1".

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