Right. If you wanted to do it all in one script, something like the following would probably be the cleanest (untested):

# get the "output" environment from the script into a hash

my %envhash = map { chomp; split( /=/, $_, 2 ) } `bash.script`;

# now get the "ambient environment" and delete that from the hash:

for ( `bash -c env` ) {
    ( my $var, undef ) = split( /=/ );
    delete $envhash{$var};
}
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