From type/IO/Handle

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method seek(IO::Handle:D: Int:D $offset, SeekType:D $whence --> True)

Move the file pointer (that is, the position at which any subsequent read or write operations will begin) to the byte position specified by $offset relative to the location specified by $whence which may be one of:

• SeekFromBeginning: The beginning of the file.

• SeekFromCurrent: The current position in the file.

• SeekFromEnd: The end of the file. Please note that you need to specify a negative offset if you want to position before the end of the file.

From type/IO/CatHandle

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method seek(IO::CatHandle:D: |c)

Calls .seek on the currently active source handle, forwarding it all the arguments, and returns the result. Returns Nil if the source handle queue may get exhausted. NOTE: this method does NOT perform any source handle switching, so seeking past the end of the current source handle will NOT seek to the next source handle in the queue and seeking past the beginning of the current source handle is a fatal error. Also see source handle queue may get exhausted, to learn the details on when source handles are switched.

(my $f1 = 'foo'.IO).spurt: 'foo';
(my $f2 = 'bar'.IO).spurt: 'bar';
 
with IO::CatHandle.new: $f1, $f2 {
    .get.say;                     # OUTPUT: «foo␤» 
    .seek: -2, SeekFromCurrent;
    .readchars(2).say;            # OUTPUT: «oo␤» 
    .seek: 1000, SeekFromCurrent; # this doesn't switch to second handle! 
    .readchars(3).say;            # OUTPUT: «bar␤» 
    try .seek: -4;                # this won't seek to previous handle! 
    say ~$!;                      # OUTPUT: «Failed to seek in filehandle: 22␤» 
}