Why not have it write all the filetypes to a ramfile first:

my %sizes;
for my $type ( Imager->write_types ) {
    open my $ramFH, '>:raw', \$buffer;
    $im->write( fh => $ramFH, type => $type );
    $sizes{ length $buffer } = $type;
}

## Pick type of smallest
my $filetype = $sizes{ (sort{ $a <=> $b } keys %sizes)[0] };

## write to real file
...
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