It's not documented very well, but you can set the file type before you create the blob, with the "magick" option.

$image->Set(magick=>'gif');  #set it to any type that's IM legal
my $blob = $image->ImageToBlob();
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Here is a working proof.

#!/usr/bin/perl
use warnings;
use strict;
use Image::Magick;

my $image = Image::Magick->new(
   size => "600x600" );

$image->Read("xc:white");
$image->Draw(
   primitive => 'line',
   points    => "300,100 300,500",
   stroke    => '#600',
);

$image->Set(magick=>'gif');

my $blob = $image->ImageToBlob();

open(FH,"> $0.gif")or die "$!\n";
print FH $blob;
close FH;
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