This is something I find I need quite often, and I usually do something like:

use Time::HiRes qw( time );  # If required

my @t;
$t[0]= time();

# do some stuff


$t[1] = time();
my $diff = formatted_time_diff(@t);

print "Processing took $diff seconds\n";

sub formatted_time_diff {
    return sprintf("%.2f", $_[1]-$_[0])
}
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Hope this helps,
Darren