The basic way of clearing a variable's taint flag is to apply the m// operator to the variable. The regex you use for the match determines the quality of your untainting.

To wrap the match in a sub, you should use the aliased arguments rather than a copy, so that the taint flag is cleared for the external variable. For example, using the unicode printable test:

sub isPrintable {
    $_[0] =~ m/^\p(IsPrint)+$/;
}
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The Regexp::Common module may be useful to you as a library. Email addresses are notoriously difficult.

After Compline,
Zaxo