You need to use the non-capturing pattern (?:foo) for the alternation unless you want it to be included in the results. 
(undef, $userid) = split(qr{(?:/|\\)}, $ENV{'REMOTE_USER'});

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Update
You actually don't need the grouping at all, and could just use alternation without parens, or could use a character class. These all work the same: 
# Alternation with non-capturing clustering (?:foo)
(undef, $userid) = split(qr{(?:/|\\)}, $ENV{'REMOTE_USER'});

# Alternation without clustering
(undef, $userid) = split(qr{\\|/}, $ENV{'REMOTE_USER'});

# Character class including '\' and '/'
(undef, $userid) = split(qr{[\\/]}, $ENV{'REMOTE_USER'});

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In reply to Re: a regex to split this... by imp
in thread a regex to split this... by s_m_b

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