Hello again, 2015_newbie,

A typical print statement has the following syntax:

print FILEHANDLE LIST
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where LIST is a list of what things to print, and FILEHANDLE is a special value telling Perl where those things are to be printed. If FILEHANDLE is omitted, the destination defaults to the last selected filehandle — typically STDOUT, which means print to the screen. If you want your output to go to a file — as you do here — then you need to open the file for writing and associate that opened file with a filehandle variable; then you give that filehandle to print to tell it where to print to.

Now, let’s return to the line:

print { $cars{$fields[4]} } $fields[0], ',', $fields[4], "\n"
    if /engineer/ && !/,-,/;
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It might make things clearer if we add a couple of variables within the if clause:

if (($fields[1] eq '1'  ||
     $fields[1] eq '0') &&
     /$pattern/)
{
    push @strings, $_;

    my $number = $fields[0];
    my $model  = $fields[4];

    print { $cars{$model} } $number, ',', $model, "\n"
        if /engineer/ && !/,-,/;
}
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Now it’s easier to see: in the LIST, the variable $model is what gets printed (second); but in the expression which denotes the filehandle, $model is the key corresponding to the filehandle stored in the hash %cars. So the value of $model (i.e., $fields[4]) might be DATSUN_BLUE (a string), but the corresponding hash entry, $cars{$model} (i.e., $cars{$fields[4]}), will be a special value — one which Perl understands — which identifies the file “tmp/DATSUN_BLUE.txt” as the file for print to write to.

It might help you to study perldata to get a good grasp of arrays and hashes.

Update: Corrected typo and improved wording.

Hope that helps,