The whole point here is to use $tee->format_write() instead of write. Then you don't have to fool around with $^ et al. (it all happens automagically). A better way for me to have shown this might have been with more calls to $tee->format_write(), as in the following modified version of what I posted earlier. 
use strict;
use warnings;
use IO::Tee;

my %hash = map { ("key$_", "val$_") } ( 1 .. 5 );
my $key;


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

format TEE =
@<<<<<  @<<<<<
$key,   $hash{$key}
.

# 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/ );

for $key ( sort keys %hash ) {
    $tee->format_write( q/main::TEE/ );
}

[download]
from the shell:

-> ./tst-tee-write 
==== Top of Output ====
key1    val1
key2    val2
key3    val3
key4    val4
key5    val5
-> cat junk.out 
==== Top of Output ====
key1    val1
key2    val2
key3    val3
key4    val4
key5    val5

[download]

In reply to Re^3: IO::Tee and write / format by broomduster
in thread IO::Tee and write / format by beadon

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