Programming Perl, 4th Edition

select (output filehandle)

select FILEHANDLE
select

For historical reasons, there are two select operators that are totally unrelated to each other. (See the next section for the other one.) This version of the select operator returns the currently selected output filehandle and, if FILEHANDLE is supplied, sets the current default filehandle for output. This has two effects: first, a write or a print without a filehandle will default to this FILEHANDLE; second, special variables related to output will refer to this output filehandle. For example, if you have to set the same top-of-form format for more than one output filehandle, you might do the following:

select REPORT1;
$^ = "MyTop";
select REPORT2;
$^ = "MyTop";

But note that this leaves REPORT2 as the currently selected filehandle. This could be construed as antisocial, since it could really foul up some other routine’s print or write statements. Properly written library routines leave the currently selected filehandle the same on exit as it was on entry. To support this, FILEHANDLE may be an expression whose value gives the name of the actual filehandle. Thus, you can save and restore the currently selected filehandle like this:

my $oldfh = select STDERR;
$| = 1;
select $oldfh;

or idiomatically but somewhat obscurely like this:

select((select(STDERR), $| = 1)[0])

This example works by building a list consisting of the returned value ...