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  $s1 = shift;
    my @ar = split (" ",$s1);

    my @dcomp;

    push @dcomp, "$ar[0] $ar[1] $ar[2]";
    foreach  my $i ( 2 .. $#ar-1  )
    {
        if ($ar[$i] =~ /[A-Z]+/)
        {
            push @dcomp, "$ar[$i] $ar[$i+1] $ar[$i+2]";
        }
    }

    print Dumper \@dcomp;
    return \@dcomp;
}