Depending on what exactly you need this for, you could maybe transform all keys to the same case (e.g. lower-case) before you put the entries in the hash — in which case you can then do a direct lookup.

This code will catch duplicate IDs that are identical.

#!/usr/bin/perl -w
#<SortPW2.pl> /etc/passwd sort by ID

my $PASSWD="passwd";
my $fpid = open(PFILE, "<$PASSWD") or die "Password File Not Found: $!
+\n";

my %ids;
while (my $line = <PFILE>) {
   @ID = split(/:/, $line);
   my $id = ${ID[0]};
   if ($ids{$id}) {
       print "'$id' is duplicate\n";
   } else {
      $ids{$id} = 1;
   }
}
[download]

I wish also to catch IDs that differ only in case. (ie. root , Root , ROOT) Thanx.

This is tangentially discussed in How can I remove duplicate elements from a list or array?, a perlfaq. You can construct your hash, and output whenever you hit a duplicate:

my %seen;
while (my $line = <PFILE>) {
   @ID = split(/:/, $line);
   my $id = $ID[0];
   if ($seen{lc($id)}++) {
       print "'$id' is duplicate\n";
   }
}
[download]

Note you had incorrect syntax for accessing a list element. I would also recommend against using variables with the same character sequence but different cases for names.

Just lowercase the $id you use for the hash lookup:

   my $id = $ID[0];
   my $id_lc = lc $id;
   if ($ids{$id_lc}) {
       print "'$id' is duplicate\n";
   } else {
      $ids{$id_lc} = 1;
   }
[download]

my %hash = (
    fOO  =>  1,
    FoO  =>  2,
    FOo  =>  3,
);
[download]

?node_id=3989;HIT=hash%20case;re=N

I suggest that you always create a hash-key that consists of (say) lowercased letters.   Then, if necessary, let each hash-element point to an arrayref.   Within each element in that array is the unadulterated key.

So, the hash serves its intended purpose:   now, as you iterate through the array that you have found, you know each one has a nearly-matching key value.