Programming Perl, 4th Edition

Format Variables

The current format name is stored in the variable $~ ($FORMAT_NAME), and the current top-of-form format name is in $^ ($FORMAT_TOP_NAME). The current output page number is stored in $% ($FORMAT_PAGE_NUMBER), and the number of lines on the page is in $= ($FORMAT_LINES_PER_PAGE). Whether to flush the output buffer on this handle automatically is stored in $| ($OUTPUT_AUTOFLUSH). The string to be output before each top of page (except the first) is stored in $^L ($FORMAT_FORMFEED). These variables are set on a per-filehandle basis, so you’ll need to select the filehandle associated with a format in order to affect its format variables:

select((select(OUTF),
        $~ = "My_Other_Format",
        $^ = "My_Top_Format"
       )[0]);

Pretty ugly, eh? It’s a common idiom, though, so don’t be too surprised when you see it. You can at least use a temporary variable to hold the previous filehandle:

$ofh = select(OUTF);
$~ = "My_Other_Format";
$^ = "My_Top_Format";
select($ofh);

This is a much better approach in general because not only does legibility improve, but you now have an intermediary statement in the code to stop on when you’re single-stepping in the debugger. If you use the English module, you can even read the variable names:

use English;
$ofh = select(OUTF);
$FORMAT_NAME     = "My_Other_Format";
$FORMAT_TOP_NAME = "My_Top_Format";
select($ofh);

But you still have those funny calls to select. If you want to avoid them, use the IO::Handle module bundled with Perl. Now you can access these special ...