Here's a couple very common example use cases for shift, unshift, push. The @INC array is special in Perl, and is used to store directories where libraries are housed. To add a custom location, it's common to unshift @INC

The most common use of shift is extracting parameters from the special @_ array within a subroutine. @_ in Perl is the default array, and is implicitly used to store all parameters going into a subroutine. In the shift line I don't say shift @_;, because the @_ is implicit.

In the last example, I use push to add elements to a new array based on contents from a different array.

#!/usr/bin/perl

# unshift

print "$_\n" for @INC;
unshift @INC, '/home/steve/my_lib_dir';
print "$_\n" for @INC;

# shift

parameter("Hello, world!");

sub parameter {
    my $param = shift;
    print "$param\n";
}

# push

my @numbers = qw(1 2 3 4 5);
my @totals;

for my $elem (@numbers){
    push @totals, $elem + 5;
}

print "$_\n" for @totals;
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-stevieb