As someone else pointed out, you should check for errors. Not only with $graph->plot but also with open, binmode, print and close. These all return undef on failure and have the error in $!.

open TMPFILE, ">file.png"    or die $!;
binmode TMPFILE              or die $!;
print TMPFILE $db_image->png or die $!;
close TMPFILE                or die $!;
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This is the bare minimum of error checking and reporting that you should do. It is perhaps a good idea to make full sentences: "Could not open file.png: $!\n".

Alternatively, you can use the Fatal module to wrape open, binmode and close:

use Fatal qw(open binmode close);

open my $tmpfile, ">file.png";
binmode $tmpfile;
print $tmpfile $db_image->png or die $!;
close $tmpfile;
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Note that I used a lexical filehandle here. Globals are bad for many reasons. See Coping with scoping for information about that.

When you have done all this, it's time to take indentation lessons :) In fact, you should probably just get a good Perl book or tutorial to learn from. (Like Beginning Perl or this tutorial or one of the many non-free resources out there.)