%ENV is global, so it depends on whether you have localized %ENV or not.

$ENV{foo} = 'outside';
{
    local %ENV;
    $ENV{foo} = 'inside';
    print `echo \$foo`;
}
print `echo \$foo`;
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2;0 juerd@ouranos:~$ perl -e'undef christmas'
Segmentation fault
2;139 juerd@ouranos:~$
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