...beware of newlines

This should be a sufficient warning to avoid such a solution. There's no chance to find out whether the following env output comes from two environment variables FOO and PERL5LIB or just from setting one variable with export FOO=$(echo -e "bar\nPERL5LIB=my/malicious/stuff"):

FOO=bar
PERL5LIB=/my/malicious/stuff
[download]

On Windows, the same misleading output can be created by typing the following (where <enter> is the "enter" key):

SET FOO=BAR^<enter>
<enter>
PERL5LIB=c:/my/malicious/stuff<enter>
[download]