If you are going to untaint in this method ("anything is good"), why bother with taint checks at all.

Because in Perl, taintness is all or nothing. If you turn it on for a Perl program, anything coming from the outside is marked tainted - even data coming from untrusted sources, or data that you are going to use in a way that will be safe regardless what it contains. But if you to turn on taint checking because there's one untrusted source, of which you'll be using some data in a potentially insecure way, you have all your data mark tainted.

Ah, if only Perl had a way to mark data from specific sources to be tainted.

    sysopen my $handle, "untrusted_file", O_RDONLY | O_TAINTED or die;
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