How about using tv_interval? ...

#!/usr/bin/perl -w                                    
                                           
use strict;
use warnings;

use Time::HiRes qw( gettimeofday tv_interval );

my $start = [gettimeofday];

my $line;

$line++ while (<>);   # Count number of lines in the file(s)

my $end = [gettimeofday];
my $difference = tv_interval($start, $end);

print "It took ", $difference, " second\n";
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which gives something like:

It took 0.000103 second
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