Question

I found this in Mail::IMAPClient. Where does the $_ in $SEARCH_KEYS{ uc($_) } come from?

sub _quote_search {
    my ( $self, @args ) = @_;
    my @ret;
    foreach my $v (@args) {
        if ( ref($v) eq "SCALAR" ) {
            push( @ret, $$v );
        }
        elsif ( exists $SEARCH_KEYS{ uc($_) } ) {
            push( @ret, $v );
        }
        elsif ( @args == 1 ) {
            push( @ret, $v );    # <3.17 compat: caller responsible for quoting
        }
        else {
            push( @ret, $self->Quote($v) );
        }
    }
    return @ret;
}

Answer 1

That looks to me like a typo where the author converted an anonymous for loop foreach (@args) to one with an explicit iterator variable foreach my $v (@args) and forgot to convert all the incidences of $_ to $v.

You should probably file a bug against the distribution on CPAN.