You need the format_write and format_top_name methods (and maybe others, depending on how sophisticated your form handling needs to be). IO::Tee inherits some format-handling code from IO::Handle. The docs for IO::Tee really only mention it in passing, but it becomes clearer in the docs for IO::Handle, and you need to be sure to tell IO::Tee specifically about things like which format to use for top-of-form (which is "automatic" for STDOUT and the like).

The following code snippet should get you started. Notice that since the name of the format is passed down to IO::Tee, you need to specify the full package name of the format (o/wise you get complaints about undefined formats from IO::Tee).

use strict;
use warnings;
use IO::Tee;

my( $abc, $xyz ) = ( q/hello/, 12345 );

format TEE_TOP =
==== Top of Output ====
.

format TEE =
@<<<<<  @#####
$abc,   $xyz
.

# with lexical file handle; can also use bareword handle
open my $outfile, '>', 'junk.out'  or die "error opening junk.out: ($!
+)";
my $tee = IO::Tee->new( \*STDOUT, $outfile );

# see docs for IO::Tee and IO::Handle for explanation of these methods
# and others available for proper format use/control
$tee->format_top_name( q/main::TEE_TOP/ );
$tee->format_write( q/main::TEE/ );
[download]

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From the shell:

-> ./tst-tee-write
==== Top of Output ====
hello    12345
-> cat junk.out
==== Top of Output ====
hello    12345
[download]

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