simple regex problem

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hey guys
I'm having a problem with the following regex

$line =~ s/.{2}\s+/:/;
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All I want to do is swap any number of spaces (the first 2) with ":"
i.e.

$line = 'this has    three     spaces'; #before
$line = 'this:has:three spaces';        #after
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Any idea what I'm doing wrong?