A couple simple ways...

my $first_line = $array[0];
# or
my $first_line = shift @array; # This is destructive to @array.
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See perlintro and shift for details.

..or

  my ( $first_line ) = @array;
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This works by the fact that the parenthesis denote a list, and you're copying the elements of the array into the elements in the list (of which there is just one in this example). This is non-destructive to the array.

You can also view the second and third lines and soforth by continuing with this notion:

  my ( $first, $second, $third ) = @array;
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This is a technique I regularly use when parsing parameters passed to a function. The function parameters are placed in @_ and so I write:

  sub myfunction( $ $ $ ) {
      my ( $param1, $param2, $param3 ) = @_;

      # ...
  }
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monarch's usage is pretty standard, I think. For example,

destructive

(~ function) Capable of modifying its arguments.

That's from here.

the lowliest monk

That statement hit me too. As far as most people are concerned, a destructor should deallocate all the resources an object owns (in a loose sense, not strictly an OO object, here I consider an array as an object).

shift obviously does not meet that criteria, it is just a usual method.