while (<>) {...}    # equivalent in only while!
    while (defined($_ = <>)) {...}

    /^Subject:/
    $_ =~ /^Subject:/

    tr/a-z/A-Z/
    $_ =~ tr/a-z/A-Z/

    chop
    chop($_)

Here are the places where Perl will assume $_ even if you don't use it:

• Various unary functions, including functions like ord() and int(), as well as the all file tests (-f, -d) except for -t, which defaults to STDIN.

• Various list functions like print() and unlink().

• The pattern matching operations m//, s///, and tr/// when used without an =~ operator.

• The default iterator variable in a foreach loop if no other variable is supplied.

• The implicit iterator variable in the grep() and map() functions.

• The default place to put an input record when a <FH> operation's result is tested by itself as the sole criterion of a while test. Note that outside of a while test, this will not happen.

Source: Perl predefined variables
Copyright: Larry Wall, et al.