In the part where you have the form, you could try

print $cgi->hidden('time',time);
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then in the other part

my $time=time-param('time');
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that gives you the number of seconds as a decimal. then you can do

print $cgi->h2(sprintf("It took you %02d:%02d to take the quiz.",int($
+time/60),int($time%60)));
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Hope this helps.