Programming Perl, 4th Edition

shift

shift ARRAY
shift

This function shifts the first value of the array off and returns it, shortening the array by one and moving everything down. (Or up, or left, depending on how you visualize the array list. We like left.) If there are no elements in the array, the function returns undef.

If ARRAY is omitted, the function shifts @_ within the lexical scope of subroutines and formats; it shifts @ARGV at file scopes (typically the main program) or within the lexical scopes established by the eval STRING, BEGIN {}, CHECK {}, UNITCHECK {}, INIT {}, and END {} constructs.

Subroutines often start by copying their arguments into lexical variables, and shift can be used for this:

sub marine {
    my $fathoms = shift;  # depth
    my $fishies = shift;  # number of fish
    my $o2      = shift;  # oxygen concentration
    # ...
}

shift is also used to process arguments at the front of your program:

while (defined($_ = shift)) {
    /^[^–]/     && do { unshift @ARGV, $_; last };
    /^–w/       && do { $WARN = 1;         next };
    /^–r/       && do { $RECURSE = 1;      next };
    die "Unknown argument $_";
}

You should consider the standard Getopt::Std and Getopt::Long modules for processing program arguments.

Starting with v5.14, shift can take a reference to an unblessed array, which will be dereferenced automatically. This aspect of shift is considered experimental. The exact behavior may change in a future version of Perl.

See also unshift, push, pop, and ...