Not only 'premature optimization is the root of all evil'; not only such a microoptimization is completely meaningless; but it's actually the other way around... declaring variables inside a loop is quite a bit faster.

Really? Go figure> (It's (much) more complicated than that!):

use strict;
use warnings;

use Benchmark qw( cmpthese );

my @strings = qw(
  USERID1|2215|Jones|
  USERID1|1000|Jones|
  USERID3|1495|Dole|
  USERID2|2500|Francis|
  USERID2|1500|Francis|
);

cmpthese(
    -1,
    {
        outside => sub {
            my ( $x, $y, $z );
            for (@strings) {
                ( $x, $y, $z ) = split /\|/;
            }
        },
        outside2 => sub {
            my ( $x, $y, $z );
            for (@strings) {
                ( $x, $y, $z ) = split /\|/, 3;
            }
        },
        inside => sub {
            for (@strings) {
                my ( $x, $y, $z ) = split /\|/;
            }
        },
        inside2 => sub {
            for (@strings) {
                my ( $x, $y, $z ) = split /\|/, 3;
            }
        },
    }
);

__END__
C:\test>junk
             Rate  outside   inside  inside2 outside2
outside   58201/s       --     -38%     -71%     -73%
inside    93659/s      61%       --     -53%     -57%
inside2  197610/s     240%     111%       --     -10%
outside2 218802/s     276%     134%      11%       --
[download]

When you can explain that; then you may pontificate on the subject.