Two errors.

If the list of keys to keep is static, you want 

my $inv = qr/two|three/;

[download]

If the list of keys to keep is dynamic, you want 

my $inv = do{ local $" = q{|}; qr{@{[ map quotemeta, @keep ]}} };

[download]

or the much simpler 

my ($inv) = map qr/$_/, join '|', map quotemeta, @keep;

[download]

In all cases, use /^$inv\z/ in the grep.

In reply to Re^2: performing operation on elements not in list? by ikegami
in thread performing operation on elements not in list? by Anonymous Monk

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