Your specification of 'first 3 characters are whitespaces and/or a number followed by ":"' is a bit conflicting - does that include 3 whitespaces followed by a number? should 3 whitespaces be followed by a colon? - but here's the best match I can come up with for that description (I'm guessing the answer to both of the above is "no"):

#!/usr/bin/perl -w
use strict;

# Build a regex of "\d\d\d:|\s\d\d:|\s\s\d:|\s\s\s"
my $str = join ":|", map { '\s'x$_ . '\d'x(3-$_) } 0 .. 3;

while (<>){
    s/^$str//; # Remove anything that matches regex from start of line
    print;
}
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