You have a false impression of chomp from the looks of it. What it does is destructively removing trailing whitespace from a string, whereas you appear to be using it to split and initialise two variables. I imagine you'll want something more like this

#!/usr/bin/perl

open USRS, "users" or die "can't open file: $!";

while(<USRS>) {
  chomp;
  my($user, $pass) = split ':';

  # do stuff with $user and $pass
}

close USRS;
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That loops through the file, dropping any trailing whitespace (e.g a newline), then splits on the colon and assigns the left side of the colon to $user and the right side to $pass (the code assumes that your data will only ever look like user:pass). See. split for more info on splitting a string.
HTH

_________
broquaint