36 chars and handles blank lines, empty lines and lines starting with a colon:

@list = (
  "123 : oain:b:okfbd",
  "",
  "456",
  "foo bar    : df a dsaf asdf",
  "111 :",
  ":  adf",
  "      "
);

$, = "\n";

print map{/(.*?)(\s*:|\s*$)/;$1||()}@list;
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Chokes if the values in front of the colon is '0'.

Update: 33 chars (destructive):

map{s/\s*:.*|\s*$//;$_||()}@list;
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Update: 32 chars (destructive):

map{s/\s*(:.*|$)//;$_||()}@list;
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