I would have thought the same. But you could get around it by building your RE as a string and then putting that into the s///: 
my $allowed = "pure";
my $re = "[^\Q$allowed\E]";

my $str = "p uuu+tr-ed";
$str =~ s/$re//g;
print $str, "\n";

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In reply to Re^3: Scrubbing a string by Roy Johnson
in thread Scrubbing a string by princepawn

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