I think you probably mean 'split' rather than 'chomp'. In fact you probably want both. Chomp removes the new line character from the end of the line. Split divides it up using a particular delimiting character or pattern. Try this:

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

    # do stuff with $user and $pass
}
close USRS;
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