#!/usr/bin/perl --
use strict;
use warnings;
use Data::Dump qw/ dd /;
my $line = "k1:v1  k2:v2   k3:v3";
my %pairs;
while( $line =~ m{\b([^:\s]+):([^:\s]+)\b}g ){
    my( $k, $v ) = ( $1, $2 );
    $pairs{$k}=$v;
}
dd( \%pairs );
if( delete $pairs{'k3'} ){
    %pairs = ( k3 => { %pairs } );
}
dd( \%pairs );
__END__
{ k1 => "v1", k2 => "v2", k3 => "v3" }
{ k3 => { k1 => "v1", k2 => "v2" } }