poj has shown you how to open a normal file. The previous code that used DATA is using the special filehandle DATA, which reads from beyond the __DATA__ marker, as if it were a separate, already-opened file. For more on the __DATA__ special literal, and the automatic DATA filehandle, see https://perldoc.perl.org/perldata.html#Special-Literals

edit: clarify

In reply to Re^3: Extract length sum from columns and print pattern into another file by pryrt
in thread Extract length sum from columns and print pattern into another file by rebkirl

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