• $_ is aliased to $x by the foreach loop,
• while does not localize $_ (like foreach loops do), and
• while (<FH>) assigns to $_.

Fix:

sub readZip {
   local $_;
   open local *FH, '/dev/null';
   while (<FH>) { print "Got something!\n"; }
}
[download]

I localized FH as well.