Since Perl 5.8.0 you can open a filehandle for writing to a scalar. Then, if you make that filehandle the default, writes will go there.

my %hash = (one => 1, two => 2, three => 3);

my $output;
open FH, '>', \$output;
my $old_fh = select(FH);

my $key;
foreach $key (keys %hash) { 
    write;
}   

select ($old_fh);

print $output;

format FH =
@<<<<<<<<< @##
$key, $hash{$key}
.
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three        3
one          1
two          2
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Unfortunately, this doesn't seem to work with localized filehandles.

Writing to a scalar is a pretty good way to do it:

use strict;
use warnings;

my %hash = (one => 1, two => 2, three => 3);

# create a format picture for use with formline
format OUTPUT =
@<<<<<<<<< @##
$_, $hash{$_}
.

my $str;

open OUTPUT, '>', \$str;
write OUTPUT foreach sort keys %hash;
close OUTPUT;

print $str;
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tie won't work, unfortunately. Here's a little trick that'll let you fudge it, with thanks to runrig's snippet for the inspiration/pointer and perldoc for the details.

### see perldocs perlvar, perlform, and formline for more details
my $output;
my %hash = (one => 1, two => 2, three => 3);

# create a format picture for use with formline
my $picture = '@<<<<<<<<< @##' . $/;

foreach my $key (keys %hash) {
  # clear out the special variable $^A ("$ACCUMULATOR")
  $^A = '';
  # again, see perldoc formline for details
  formline($picture,$key,$hash{$key});
  # add the contents of $^A to $output
  $output .= $^A;
}

print $output;
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one          1
two          2
three        3
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package Catch;

sub TIEHANDLE {
    my($class, $var) = @_;
    return bless { var => $var }, $class;
}

sub PRINT  {
    my($self) = shift;
    ${'main::'.$self->{var}} .= join '', @_;
}

sub OPEN  {}    # XXX Hackery in case the user redirects
sub CLOSE {}    # XXX STDERR/STDOUT.  This is not the behavior we want
+.

sub READ {}
sub READLINE {}
sub GETC {}


package main;

# pre-5.8.0's warns aren't caught by a tied STDERR.
$SIG{__WARN__} = sub { $main::_STDERR_ .= join '', @_; };
tie *STDOUT, 'Catch', '_STDOUT_' or die $!;
tie *STDERR, 'Catch', '_STDERR_' or die $!;
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Oops - Test::Inline

--MidLifeXis