parth00:

I don't see your bug, but why not just use something like (untested):

sub populatemessagedefs {
        my $hashref = shift;
        my $string = shift;
        my @list = split(',', $string);
        while (@list) {
                my $k = shift @list;
                my $v = shift @list;
                $$hashref{$k}=$v unless exists($$hashref{$k}) and $v <
+= $$hashref{$k};
        }
        my %hash2 = split(',', $string);
}
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Update 2: I missed the bit where you're keeping the largest value for each key. Corrected code above, original code below.

sub populatemessagedefs {
    my $hashref = shift;
    my $string = shift;
    %$hashref = (%$hashref, split(',',$string));
}
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Update: I forgot to mention--you don't want to use a prototype on your subroutine definition.

...roboticus

Is it always zero at the beginning of the loop body, or on every other iteration?  In case of the latter, it could be because the conditions you're testing are such that $i gets decremented in the elsif branch every time it has become 1.  Heavily simplified, like this:

my $i=0;

foreach my $field (qw(foo bar baz quux)) {

    print "i=$i\n";
    
    if ($i == 0) {
        $i++;                 # increment
    }    
    elsif ($i == 1) {
     
        if (0) {
            if (0) {
                $i--;
            }    
        }
        else {
            $i--;             # decrement   
        }            
    }    
}

__END__
i=0
i=1
i=0
i=1
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#!/usr/bin/perl
use strict;
use warnings;

&populatemessagedefs(undef, '1,2,3,4,5,6');

sub populatemessagedefs()
{
    my ($ref_to_msg_defs,$Everythingelse) = @_;            
    my $temp_value;
    my $temp_key;
    my @Interface_Message_List = split (',',$Everythingelse);
    my $i=0;
    my %ref_to_msg_defs;
    
    foreach my $field(@Interface_Message_List) {
        print "$i\n";
        #In next iteration $i is ‘0’ again
        if ($i == 0) {
            $temp_key =  $field;
            $i++;    
            #Value of $i is 1 now        
        }    
        elsif ($i == 1) {
         
            $temp_value = $field;
            
            if (exists $ref_to_msg_defs->{$temp_key}) {
                if ($field > $ref_to_msg_defs->{$temp_key}) {
                    $ref_to_msg_defs{$temp_key} = $field;
                    $i--;
                }    
            }
            else {
                $ref_to_msg_defs{$temp_key} = $field;
                $i--;                
            }            
        }    
    }    
}
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I get

0
1
0
1
0
1
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which I'd expect. Two items of note for you:

1. You are using Prototypes incorrectly by declaring your subroutine with sub populatemessagedefs(). You expect two scalar arguments, not zero as you've written there. Unless you know what prototypes are and what they are not, you should really replace that line with sub populatemessagedefs.
2. You never use $ref_to_msg_defs, which I assume is a hash reference. Rather, in your code you use $ref_to_msg_defs{$temp_key}, which accesses the hash %ref_to_msg_defs, which is not declared in your code. In order to run under strict, I had to declare the appropriate local hash.

#!/usr/local/bin/perl
use strict;
use warnings;

my $hashRef = {};
my $string = "Tom, 18, Jack, 22, Tim, 30";
populateHash($hashRef,$string);

sub populateHash{
        my ($hashRef,$scopedString)  = @_;
        my @stringArray = split (', ',$scopedString);
        %$hashRef = @stringArray;
        return $hashRef;
        }

use Data::Dumper;
print Dumper(\%$hashRef);
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