use strict;
use warnings;

my $val1 = 5234;

my @digits = split(//,$val1);
my $string = join ('_',@digits);
$string =~ s/_(\d)$/$1/ if @digits == 4;

print $string, "\n";
#prints 5_2_34
#
#$val1 5234 prints 5_2_34
#$val1 52   prints 5_2
#$val1 523  prints 5_2_3