Have the child output the set commands instead of executing it, and process the commands in the parent.

If the parent is a Perl script: 

chomp( my @cmds = `child` );
foreach my $cmd (@cmds) {
   next unless /^set\s+(\S.*?)=(.*)/i;
   $ENV{$1} = $2;
}

[download]

If the parent is a batch file: 

for /f "usebackq delims=" %%f in (`child`) do %%f

[download]

The child would output such as 

echo set VAR=5

[download]
or 
print("set VAR=5\n");

[download]

Update: Added second snippet.

In reply to Re: Changing parent process environment variable by ikegami
in thread Changing parent process environment variable by tcf22

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