If the parent is a Perl script:

chomp( my @cmds = `child` );
foreach my $cmd (@cmds) {
   next unless /^set\s+(\S.*?)=(.*)/i;
   $ENV{$1} = $2;
}
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If the parent is a batch file:

for /f "usebackq delims=" %%f in (`child`) do %%f
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The child would output such as

echo set VAR=5
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print("set VAR=5\n");
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Update: Added second snippet.