:: test.bat
@echo off
echo FOO=foo
echo BAR=bar

##</code><code>##

for (split /\n/, `test.bat`) {
   my ($k, $v) = split /=/, $_, 2;
   $ENV{$k} = $v;
}

##</code><code>##

:: wrapper.bat
@echo off
call existing.bat
set

##</code><code>##

:: existing.bat
@echo off
set FOO=foo
set BAR=bar

##</code><code>##

for (split /\n/, `wrapper.bat`) {
   my ($k, $v) = split /=/, $_, 2;
   $ENV{$k} = $v;
}