WRT your first SSCCE:   while does not automatically assign the result of its CONDITION evaluation to  $_ (in contrast to the
    while (<FILEHANDLE>) { do_something_with($_); }
special case):

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"foreach my $filename (qw(a b c)) {
   dd 'before while loop, $filename is', $filename;
   while ($filename) {
     dd 'in while loop, $_ is', $_;
     last;
     }
   }
"
("before while loop, \$filename is", "a")

("in while loop, \$_ is", undef)

("before while loop, \$filename is", "b")

("in while loop, \$_ is", undef)

("before while loop, \$filename is", "c")

("in while loop, \$_ is", undef)
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