my $L = $tabs->[10];
while ($L)                         # ie, as long as $L has a value
{
   $m->[ $L->[1] ] = $L->[2];      # assign the 3rd value referenced by $L to the arrayref $m 
                                   # (indexing with the 2nd value referenced by $L)
   $L = $L->[0];                   # assign $L the first value of the arrayref its referencing 
                                   # (or undef if not available)
}