I saw the same outputs in my perl 5.12.3. It seems perl confusing for $1 because this also prints "100" without warnings;

%hash =(
  a => ($b =~ /(\d+)/ ? $1 : 0),
  b => "test b",
);
print "$_=#$hash{$_}#\n" for keys %hash;
[download]

And named capture seems to work fine.

%hash = (
  a => (($b =~ /(?<tag>\d+)/) ? $+{tag} : 0),
  b => (($b =~ /(?<tag>test)/) ? $+{tag} : 0),
);
print "$_=#$hash{$_}#\n" for keys %hash;
[download]

But I have no idea for why named capture doesn't confuse...