Observation: In your newer code, you had the following code in your viewer sub:

my $next_html = "http://.../albums.cgi?...&pic=$nextpic&user$user";
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You missed an equal '=' from the URL, should be user=$user.
Also in the same viewer sub, you had two lines:

my $currentpic = pop(@splitsrc);
print $currentpic;
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Where the print $currentpic; bit will print the image name to the standard out, which will certainly get printed before your content-type header, and this of cause will interfere with how the HTML is handled by the client browser. Remember that anything printed to STDOUT will get sent to the target browser.