Does this do what you want?

use 5.010;
use strict;
use warnings;

while (<DATA>) {
    chomp;
    my $copy = $_;
    s/\d\K(o+)/0 x length $1/eg;
    say "$copy -> $_";
}

__DATA__
o
1o
123ooo
12oo123ooo56
abcooo1o



o -> o
1o -> 10
123ooo -> 123000
12oo123ooo56 -> 120012300056
abcooo1o -> abcooo10
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