The following might help in getting light on the issue:

#!/usr/bin/perl

use strict;
#use warnings;

my ( undef, undef, $uid, $gid ) = getpwnam( 'roo' )
    || warn "Oh no - || and not existent user roo\n";

print "1. UID: $uid, GID: $gid\n\n";

( undef, undef, $uid, $gid ) = getpwnam( 'root' )
    || warn "Oh no - || and existent user root\n";

print "2. UID: $uid, GID: $gid\n\n";

( undef, undef, $uid, $gid ) = getpwnam( 'roo' )
    or warn "Oh no - OR and not existent user roo\n";

print "3. UID: $uid, GID: $gid\n\n";

( undef, undef, $uid, $gid ) = getpwnam( 'root' )
    or warn "Oh no - OR and existent user root\n";

print "4. UID: $uid, GID: $gid\n\n";
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If you put the code in test.pl for example, try to run then

perl -MO=Deparse ./test.pl
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(have a look on B::Deparse). It shows you how Perl understands the code you've written in each case.

Update. It isn't really something guaranteed to work, but I use to imagine '( || )' as opposed to ')or(' to keep in mind the much lower precedence of 'or'. And whenever I'm in doubt, I prefer to place the parens explicitely. It's not something to be ashame of ;)