First, if it's taking one to two seconds to split a line into an array, you have other problems! Also, you'll have to run your tests more than three times each to really benchmark things.

Maybe look at Devel::NYTProf and/or Benchmark.

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

use Benchmark qw[ timethese :hireswallclock];

sub with_my {
  while (<DATA>) {
    my @array = split;
  }
}

my @array2;
sub without_my {
  while (<DATA>) {
    @array2 = split;
  }   
}

for (1 ..5) {
  timethese (5000000, {
    with_my    => 'with_my()',
    without_my => 'without_my()'
  }); 
}

__DATA__
The quick brown fox jumps over the lazy dog.
[download]

The results seem to show that the difference is negligible, IMO.

Benchmark: timing 5000000 iterations of with_my, without_my...
   with_my: 11.0969 wallclock secs ( 6.75 usr +  3.42 sys = 10.17 CPU)
+ @ 491642.08/s (n=5000000)
without_my: 10.6874 wallclock secs ( 6.84 usr +  3.41 sys = 10.25 CPU)
+ @ 487804.88/s (n=5000000)
Benchmark: timing 5000000 iterations of with_my, without_my...
   with_my: 10.5256 wallclock secs ( 6.83 usr +  3.44 sys = 10.27 CPU)
+ @ 486854.92/s (n=5000000)
without_my: 10.759 wallclock secs ( 7.02 usr +  3.52 sys = 10.54 CPU) 
+@ 474383.30/s (n=5000000)
Benchmark: timing 5000000 iterations of with_my, without_my...
   with_my: 11.1877 wallclock secs ( 7.40 usr +  3.72 sys = 11.12 CPU)
+ @ 449640.29/s (n=5000000)
without_my: 10.8896 wallclock secs ( 7.20 usr +  3.63 sys = 10.83 CPU)
+ @ 461680.52/s (n=5000000)
Benchmark: timing 5000000 iterations of with_my, without_my...
   with_my: 11.0768 wallclock secs ( 7.33 usr +  3.69 sys = 11.02 CPU)
+ @ 453720.51/s (n=5000000)
without_my: 10.9737 wallclock secs ( 7.28 usr +  3.65 sys = 10.93 CPU)
+ @ 457456.54/s (n=5000000)
Benchmark: timing 5000000 iterations of with_my, without_my...
   with_my: 11.0938 wallclock secs ( 7.35 usr +  3.70 sys = 11.05 CPU)
+ @ 452488.69/s (n=5000000)
without_my: 11.018 wallclock secs ( 7.29 usr +  3.69 sys = 10.98 CPU) 
+@ 455373.41/s (n=5000000)
[download]