use Data::Dumper;

my $s1  = 'X -4 Y 3 Z';
my $s2  = 'W 1 X -4 Y 3 Z';
my $s3  = 'X 2 Y';
my $s4  = 'A -4 B -4 C -4 A -4 B';

decomp_str($s1);
decomp_str($s2);
decomp_str($s3);
decomp_str($s4);


sub decomp_str
{
    my @dcomp;
    $_ = shift;

    while( /^(\S\s+\S+\s+)(\S)(.*)$/ ) {
        push @dcomp, $1 . $2;
        $_ = $2 . $3;
    }

    ( length() == 1 ) or die( "Syntax error: $_\n" );
    print Dumper \@dcomp;
    return \@dcomp;
}