(Um, for the above to do what the op wants, I think you need something like  push @ret, "@_[$i..$i+2]"; instead of  push @ret, [$_[$i], $_[$i+1], $_[$i+2]];)

Here's my shifty suggestion:

sub decomp {
    @_ = split / /, shift;
    return [ map { join " ", (shift, shift, $_[0]) } (1..@_/2) ];
}
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update: shorter and generalizable to producing any size tuple:

sub decomp {
    my $n = 2; # for n+1-uples
    @_ = split / /, shift;
    return [ map { $_*=$n; "@_[$_..$_+$n]" } 0..@_/$n-1 ];
}
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