Sounds like you're on the right track. You are using strict, aren't you? Perhaps you have a typo. This program:

use strict;
my $rest = 'a b c d password';
my @rest = split(/ /, $rest);
print "\@rest=@rest\n";
my $apass = pop @rest;
print "PASS: $apass\n";
print "\@rest=@rest\n";
[download]

produces:

@rest=a b c d password
PASS: password
@rest=a b c d
[download]

update: And maybe you have a space at the end of your scalar, and so the last element is empty. You may want to print out the scalar first with something around it like print "($rest)\n" so you can see it.