The Perl special variable @_ is the list of parameters passed to the sub. So:

hello ('Hello', 'world');

sub hello {
    my $hi = shift;
    my $place = shift;

    print "$hi $place\n";
}
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Prints:

Hello world
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Although many people would write the sub as:

sub hello {
    my ($hi, $place) = @_;

    print "$hi $place\n";
}
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to use list assignment rather then shifting out multiple arguments. An even more sophisticated technique is to use "named" parameters by assigning to a hash instead of the positional technique just shown:

hello (hi => 'Hello', place => 'world');

sub hello {
    my %params = @_;

    print "$params{hi} $params{place}\n";
}
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Further reading: perlsub, perlop and perlfunc.