$_ is the default variable, when you don't supply one.  It can be used as the default to many functions, as well as the default value in numerous contexts, and is kind of similar to the English pronoun "it".

For example, you could either do:

foreach my $value ('1', '2', '3') {
    print "Next value is $value\n";
}
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Or you could do, instead:

foreach ('1', '2', '3') {
    print "Next value is $_\n";
}
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In contrast, @_ is the default array.  One place where it's used quite extensively is in a subroutine, to refer to passed arguments.

For example:

sub do_something {
    my ($arg1, $arg2) = @_;
    printf "You passed arguments '%s' and '%s'\n", $arg1, $arg2;
}
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To begin to get acquainted with hashes (and a lot more), please read perlintro for starters.