You are viewing the version of this documentation from Perl blead. This is the main development branch of Perl. (git commit 3931cc0434324ecd6ac7b98e28e15e92b0c99293)
IO::Handle->format_line_break_characters(EXPR)
$FORMAT_LINE_BREAK_CHARACTERS
$:

The current set of characters after which a string may be broken to fill continuation fields (starting with ^) in a format. The default is " \n-", to break on a space, newline, or a hyphen.

You cannot call format_line_break_characters() on a handle, only as a static method. See IO::Handle.

Mnemonic: a "colon" in poetry is a part of a line.