The central error that leads to the behaviour of your code are the following two lines:

...
foreach(@arr) {
    
    @IDINFO = split(/:/,"$arr[$_]");
...

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You should see lots of warnings by Perl, because $arr[$_] will always be $arr[0], because $_ is not what you might think it is.

As a first approach to finding such errors yourself, I would suggest printing $_ and $arr[ $_ ] to make sure that what you see is what you think there should be.

As a first fix, use $_ instead of $arr[ $_ ].

In general, there are many things that you can improve in your program to make it more resilient and not rely on features that were superseded by features introduced in Perl 5:

$HAN = "handle";
$FILE = "passwd.txt";
open("$HAN", "$FILE")||die "Error opening file: $!\n";

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Please do not use global filehandles. Whatever textbook promotes such use, it is hopefully very old, as this is a technique used in Perl 4. Using this technique relies on the idea that nobody nowhere in your program will ever use a filehandle with a similar name. The Perl5 way is to simply use lexical filehandles:

open my $HAN, $FILE
    or die "Error opening file '$FILE': $!";

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Now, your file iteration logic is also problematic because it mixes two approaches:

while(<$HAN>) {
@arr = <$HAN>;
}

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The while(<$HAN>) { reads one line from $HAN and stores it in $_.

The @arr= <$HAN> reads all remaining lines from $HAN and stores them in @arr.

You should use either one of the two idioms but not mix them.

...
    print "UID is: @IDINFO[2] \n";
    print "GID is: @IDINFO[3] \n";
...

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The common way to write these is to use $ instead of @:

    print "UID is: $IDINFO[2] \n";
    print "GID is: $IDINFO[3] \n";

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In reply to Re: traverse through passwd file, and split fields to extract UID/GID by Corion
in thread traverse through passwd file, and split fields to extract UID/GID by intoperl

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