You can always send your variables through the wash with a function like this:

sub default
{
        foreach (@_)
        {
                $_ = '' unless (defined($_));
        }
}
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So, in practice, this is used in a straight-forward way:

my $aaa = 'bar';
my $bbb = 0;
my $ccc = undef;
my %ddd = ( eee => undef, fff => 1 );
my @iii = (undef, 1, undef, 2);

default($aaa,$bbb,$ccc,%ddd,@iii);

print "aaa=$aaa\n";
print "bbb=$bbb\n";
print "ccc=$ccc\n";
print "ddd=eee($ddd{eee}) fff($ddd{fff})\n";
print "iii=@iii\n";
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No warnings because everything that was undef is now set to an empty string.

You could always substitute 0 instead of '' in the default routine.