Being lazy, I would abuse the power of regex's and say:

my $string = '2 0 1 2 - 7 - 2 7  9 : 3 7 : 3 1';

$string =~ s/  /x/g;
$string =~ s/ //g;
$string =~ s/x/ /g;

print $string;
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just did it out in steps.. since you want to remove all the spaces I put a spot holder where all the double spaces are supposed to be, then later replaced the 'x' with ' '. perhaps give tr/// a look, that switches out sets but I'm not sure how to switch out spaces with it.