$ perl -le'
my @x = ( 5234, 523, 52, 5 );
my $limit = 2;
for my $x ( @x ) {
    print $x;
    my $y = $limit;           
    $x =~ s/(?<=\d)(?=\d)/ $y-- ? "_" : "" /eg;
    print $x;
    }
'
5234
5_2_34
523
5_2_3
52
5_2
5
5