Just use grouping, an alternation and a negative lookahead

my $pat = qr/
  SW
  (?: # the grouping + alternation
      [:]
    |
      =? (?!ITCH) # ignore SWITCH
  )
  \s*
  ([^\s;,\n\r]+)
/x;

my %tests = (
  'SW: foo' => 1,
  'SW= bar' => 1,
  'SW  baz' => 1,
  'SWITCH'  => 0,
);

while(my($test,$res) = each %tests) {
  print "result: ", ($res ? $1 : 'PASS'), "\n"
    if $res == $test =~ $pat;
}

__output__

result: PASS
result: baz
result: foo
result: bar
[download]

See. perlre for more info.