Is there a reason for the \$s1 stuff? It seems to work just fine without....

#!perl -l
my $s1  = q/X -4 Y 3 Z/;
my $s2  = q/W 1 X -4 Y 3 Z/;
my $s3  = q/X 2 Y/;
my $s4  = q/A -4 B -4 C -4 A -4 B/;
for ( $s1, $s2, $s3, $s4 ) {
    print for /(?=(\S+\s+-?\d+\s+\S+))/g;
}
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Don't forget that for aliases its iterator, which is basically a funny kind of reference that doesnt need to be dereferenced.