When you use the g modifier, it keeps track of its last match. In the first RE (scalar context), 456 is matched: your count indicates a single match (see code below for actual value). In the second RE (list context), the remaining four sets of digits are matched. You can use pos to reset the last match position.

$ perl -Mstrict -Mwarnings -e '
> my $a="  456 789 123 456 789  ";
> my $count;
> $count = $a =~ /\d+/g ;
> print "$count\n";
> print "${^MATCH}\n";
> pos($a) = 0;
> my @a = $a =~ /\d+/g ;
> $count = @a;
> print "@a\n";
> print "$count\n";
> '
1
456
456 789 123 456 789
5
[download]

${^MATCH} is described in perlvar.