Your first regex is done on $shell, but the second two are done on $_. I think you want to take an approach like:

if ($shell ne "" and $shell !~ /false|null/) {
  # it's ok
}
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You could, of course, keep the other two regexes separate:

if ($shell ne "" and $shell !~ /false/ and $shell !~ /null/) {
  # it's ok
}
[download]

Also, the pattern // does NOT match empty strings -- it uses the last matched pattern again:

"foo" =~ /foo/;
print grep //, qw( bar foobar );  # foobar
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To match empty strings, either use /^$/ or /^\z/, depending on your definition of empty.

japhy -- Perl and Regex Hacker