Perhaps this will do the job?

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
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Updated: Re question from OP

I am just trying to demonstrate a technique. Once you see how this works, you can adapt it for your exact specific needs. Like maybe if @digits is >4, that is an error or whatever. A subroutine would be like this (shamelessly plugging my code into toolic's loop structure, thanks toolic!): (this is 2nd update)

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

sub append 
{
    my ($val, $character) = @_;
    my @digits = split(//,$val);
    my $string = join ($character,@digits);
    $string =~ s/$character(\d)$/$1/ if @digits >= 4;
    return $string;
}
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