This has been covered a few times. Check out:

• To push and pop or not to push and pop?
• Shift, Pop, Unshift and Push with Impunity!
• Speeding up unshift

Thankyou. The statement in that second link about it being an O(1) operation is exactly what I was looking for--I was afraid it might be O(n).

#!/usr/bin/perl

use warnings;
use strict;

use Benchmark;

timethese(1_000_000, {
  'use_shift' => sub { sub_with_shift(0..9) },
  'use_list' => sub { sub_with_list(0..9) },
  'use_direct' => sub { sub_with_direct(0..9) },
});

sub sub_with_shift
{
  my $sum = 0;
  while (@_)
  {
    $sum += shift;
  }
  $sum;
}

sub sub_with_list
{
  my(@a)=@_;
  my $sum = 0;
  $sum += $_
    for @a;
  $sum;
}

sub sub_with_direct
{
  my $sum = 0;
  $sum += $_
    for @_;
  $sum;
}
[download]

Benchmark: timing 1000000 iterations of use_direct, use_list, use_shift...
use_direct:  7 wallclock secs ( 6.48 usr + -0.01 sys =  6.47 CPU) @ 154559.51/s (n=1000000)
  use_list: 10 wallclock secs ( 9.85 usr +  0.06 sys =  9.91 CPU) @ 100908.17/s (n=1000000)
 use_shift:  6 wallclock secs ( 6.48 usr +  0.01 sys =  6.49 CPU) @ 154083.20/s (n=1000000)

#!/usr/bin/perl

use warnings;
use strict;

use Benchmark;

timethese(1_000_000, {
  'use_shift' => sub { sub_with_shift(0..2) },
  'use_list' => sub { sub_with_list(0..2) },
  'use_direct' => sub { sub_with_direct(0..2) },
});

sub sub_with_shift
{
  my ($one, $two, $three) = (shift, shift, shift);
  my $sum = $one+$two+$three;
}

sub sub_with_list
{
  my ($one, $two, $three) = @_;
  my $sum = $one+$two+$three;
}

sub sub_with_direct
{
  my $sum = $_[0] + $_[1] + $_[2];
}
[download]

i think my example better reflects the core of the problem

Sorry i forgot result 
Benchmark: timing 1000000 iterations of use_direct, use_list, use_shif
+t...
use_direct:  0 wallclock secs ( 0.57 usr +  0.00 sys =  0.57 CPU) @ 17
+54385.96/s (n=1000000)
  use_list:  1 wallclock secs ( 0.86 usr +  0.00 sys =  0.86 CPU) @ 11
+62790.70/s (n=1000000)
 use_shift:  2 wallclock secs ( 0.96 usr +  0.00 sys =  0.96 CPU) @ 10
+41666.67/s (n=1000000)
[download]

Besides style, is there any performance penalty paid by shift?

Yes; if you do it repeatedly hundreds of thousands of times in a tight loop, you might slow down your program as much as performing one IO operation. In other words, none that you will ever notice.