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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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Can't use an undefined value as %s reference

(F) A value used as either a hard reference or a symbolic reference must be a defined value. This helps to delurk some insidious errors.

ISBN 9781906966027Perl Language Reference ManualSee the print edition