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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.