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.