I didn't know it used a temp file. If I were to build the module to do this I'd use Tie::Handle and overload the writing functions.

I didn't test this or even try to compile it.

my $data = tie $fh, 'Tie::Handle::Scalar::WithoutTemp';

print $fh "Hello world";
print $$data;

package Tie::Handle::Scalar::WithoutTemp;
use base 'Tie::Handle';

sub TIEHANDLE {
    my $classname = shift;
    my $data;
    bless \ $data, $classname;
}

sub WRITE {
    my ( $self, $scalar, $length, $offset ) = @_;
    $$data .= substr $scalar, $offset, $length;
    1;
}

sub PRINT {
    $$data .= join $,, @_[ 1 .. $#_ ];
    1;
}

sub PRINTF {
    $$data .= sprintf $_[1], @_[ 2 .. $#_ ];
    1;
}
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