You said:

my $x = 'abc12345';
my $length = length ($x =~ /(\d+)$/);
# returns 1, the length of the number in the number of elements
[download]

That's not true. length() imposes scalar context on its argument, so the pattern match happens in scalar context and returns "1" on success and "" on failure. Thus, you're getting length(1), which is 1, not "length of the number in the number of elements", because "number of elements" is not what the regex is returning.