That is easy, the line is confusing :)

It is also confusing $dataset for a hashref

perltidy

my @results = ( $col[0], $col[3] );
for my $dataset (@data) {
    push @results, (
        sort {
            my $diff_a = $col[2] - $a->[1];
            $diff_a *= -1 if $diff_a < 0;
            my $diff_b = $col[2] - $b->[1];
            $diff_b *= -1 if $diff_b < 0;
            $diff_a <=> $diff_b;
          } @{ $dataset->{ $col[0] } }
    )[0]->[2];
}
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my @results = ( $col[0], $col[3] );
for my $dataset (@data) {
    my ( $first ) = 
        sort {
            my $diff_a = $col[2] - $a->[1];
            $diff_a *= -1 if $diff_a < 0;
            my $diff_b = $col[2] - $b->[1];
            $diff_b *= -1 if $diff_b < 0;
            $diff_a <=> $diff_b;
          } @{ $dataset->{ $col[0] } }

    push @results, $first->[2];
}
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my @results = ( $col[0], $col[3] );
for my $dataset (@data) {
    my @beef = @{ $dataset->{ $col[0] } };
    @beef = 
        sort {
            my $diff_a = $col[2] - $a->[1];
            $diff_a *= -1 if $diff_a < 0;
            my $diff_b = $col[2] - $b->[1];
            $diff_b *= -1 if $diff_b < 0;
            $diff_a <=> $diff_b;
          }   @beef;

    push @results, $beef[0]->[2];
}
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$a and $b are globals, they're how sort works

Is it still confusing?

Hi anonymous monk, yes I'm still finding this confusing. You mentioned it is confusing $dataset for a hashref. I've been playing around with this for a while and just keep getting syntax errors.

Hi anonymous monk, yes I'm still finding this confusing.

Well hold onto your pants :)

You mentioned it is confusing $dataset for a hashref.

Look at the @beef equivalent example, it is this line that dies with Can't use an undefined value as an ARRAY reference

 my @beef = @{ $dataset->{ $col[0] } };
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This line is really

my $key  = $col[0] ;
my $meat = $dataset->{ $key };
my @beef = @{ $meat };
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I misspoke when I said it is confusing $dataset for a hashref , in actuality, $dataset is a hashref.

The problem is, $key is not in $dataset, so $meat is undef

Since meat is undef, trying to treat $meat as an array by de-referencing it, triggers a warning if you have warnings on, and triggers an error, if you have strict on.

## no warnings or errors
$ perl -e "print @{ undef() }; print 6"
6

## a warning is issued, but 6 still gets printed
$ perl -we "print @{ undef() }
Use of uninitialized value in array dereference at -e line 1.
6

## with strict the warning is fatal, 6 not printed cause program died
$ perl -Mstrict -we "print @{ undef() }
Can't use an undefined value as an ARRAY reference at -e line 1.
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See References quick reference

To avoid this error, you might add

next unless $meat;
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You don't want to disable strict :)

Lets take apart a simpler example, one that builds @data

Read more... (2 kB)