G'day morgon,

"it only prints "hubba2", because (I assume) the second match only starts where the first match matched (ie pos is not reset)."

Yes, your assumption is correct:

$ perl -Mstrict -Mwarnings -E '
my $s = <<end;
hubba1
bubba
hubba2
end

$s =~ /bubba/gm;                  
say pos($s);
print $s =~ /(hubba\d)/gm;
'
12
hubba2
[download]

[length("hubba1\nbubba") == 12]

and agrees with the pos documentation:

"Returns the offset of where the last m//g search left off ..."

In your last statement, you appear to be contradicting your previous, correct assumption:

"... my understanding is that the behaviour in the second case is a bug or is there something I don't understand?"

The code you've posted in the third case does not give "the correct behaviour"; it gives a syntax error. You probably meant /.../ instead of s/.../ (2 instances). Assuming you did, that also agrees with the pos documentation:

"... search position is reset (usually due to match failure ..."