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while( @letters > 2 ){ while( @dirs ){ ... while( @ARGV ){ while( @nodes ){ ...
$ perl -MData::Dump -e " my @f = ( 1, 0, undef, 6 ); while( my($q)=sh +ift @f ){ dd( $q ); $r++; die if $r > 10; } " 1 0 ... undef undef Died at -e line 1.