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

##</code><code>##

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