The variable "$_". (The dot is just the end of sentence period.)

$_ = "abc";
print /b/ ?1:0,"\n";  # 1

$_ = "def";
print /b/ ?1:0,"\n";  # 0
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Does it means "this" in perl?

Yes and no.

There's nothing intrinsically special about $_. It's a global variable that happens to be used by default by many operators. Two common loop structures set it by default: it's for's default iterator, and while (<$fh>) is short for while (defined($_ = <$fh>)).

(Java's?) this cannot be set, and is only used by one operator (object dereference).

$self is usually used for this in Perl, but it's just an ordinary variable. It needs to be specified explicitly, so it can have any name.