I meant adjacent digits.

To be true my problem was more complex than just matching digits. I wanted to find all possible matches for any regexp.
Take
print $_, ", " for ('1aaab2' =~ /(a+b)/g)
#prints aaab,

It prints only one result, instead of list (what's needed): 'ab', 'aab', 'aaab'.
Solution given by Corion works perfectly in this case:
print $_, ", " for ('1aaab2' =~ /(?=(a+b))/g)
#prints aaab, aab, ab,

But it has one side effect. See an example:
while ('1234' =~ /(\d\d)/g) {
print "$`<$&>$'", ", ";
}
#prints <12>34, 12<34>,

Extended regexp:
while ('1234' =~ /(?=(\d\d))/g) {
print "$`<$&>$'", ", ";
}
#prints <>1234, 1<>234, 12<>34,

So side effect is that this extended regexp doesn't allow to use $`, $&, $' variables as usually.

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