my ( $first, @list ) = @{my_list};
treat_1( $first );
treat_others( $_ ) for @list;
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If $my_list can be destroyed, it can be made shorter.

treat_1( shift @$my_list );
treat_others( $_ ) for @$my_list;
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If it cannot be destroyed, it can still be done without the temporary variables:

treat_1( $my_list->[0] );
treat_others( $my_list->[$_] ) for ( 1 .. $#{$my_list} );
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Update: Added a couple of more different implementations.

Before you enter the loop you set my $first=1 and in the if-branch you do $first=0 to switch it off.

HTH! :)

my @my_list = (...);
treat_all(@my_list);

sub treat_all {
    my ( $first, @others ) = @_;
    treat_1($first);
    treat_others($_) for @others;
}
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You can use a for loop instead of foreach

for ($i = 0; $i < $#mylist ; $i++) {
   say "first element" if ( $i == 0 );
}
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Even when you want to loop over the indexes, you can still use a foreach-style loop:

foreach my $i (0 .. $#mylist) {
   say "first element" if ( $i == 0 );
}
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foreach-style loops are generally a better idea that C-style for loops. They're clearer; more declarative. It's harder to introduce an off-by-one error, and easier to spot a mistake.

toby döt ink

It's harder to introduce an off-by-one error, and easier to spot a mistake.

Indeed. Consider
    for ($i = 0; $i < $#mylist ; $i++) { ... }
in reply above:  $i < $#mylist vice  $i <= $#mylist — or did you have that in mind?

Can anyone tell me how i can obtain the first element?

To answer the question exactly as asked you can shift off the first element:

my @array = qw/one two three/;
my $x = shift @array;
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That will remove the first element from the array leaving 'two','three'. If you don't want to alter the list you can directly access the first element:

my @array = qw/one two three/;
my $x = $array[0];
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Thanks for the help everyone.

I will take a look at all the answers and see which solution to implement.

my $cnt;
for my $element (@elements) {
  unless ($cnt++) {
    # deal w/first element
    next;
  }
  # Deal w/the rest
}
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