#! perl
use strict;
use warnings;

my $string = ' 2 0 1 2 - 7 - 2 7  9 : 3 7 : 3 1 ';
#             NB: 2 spaces here ^^

# (a) Remove single spaces
1 while $string =~ s/(^|[^ ])[ ]([^ ]|$)/$1$2/g;

# (b) Squash multiple spaces down to one
$string =~ s/[ ]{2,}/ /g;

print "'", $string, "'\n";