use MIME::Base64;
# ... your GD code from above

my $imageData = $image->png;

# save $imageData to file with binmde... well you know that...
open( my $PNG, ">", $imageFilename ) or die ...;
binmode( $PNG );
print $PNG $imageData;
close( $PNG ) or die ...;

# convert $image to tk photo
my $imageData64 = MIME::Base64::encode_base64( $imageData );
my $photo = $mw->Photo(
  -format => 'png',
  -data => $imageData64,
);

# do something with $photo...