use strict;
use warnings;

for (qw(5234 523 52)) {
    print append($_ , '_'), "\n";
}

sub append {
    my $val2      = shift;
    my $character = shift;
    if ($val2 =~ /(\d{2,3})/) {
        my $in = $1;
        my $num = join $character, split //, $in;
        $val2 =~ s/$in/$num/;
    }
    else {
        return;
    }
    return $val2;
}

__END__

5_2_34
5_2_3
5_2