sysseek($fh,0,0) returned "0 but true"
sysseek($fh,42,0) returned 42
sysseek($fh,-1,0) returned undef

        undef: bool=false, str=          "", num=0 (warns: uninit x 2)
           "": bool=false, str=          "", num=0 (warns: numeric)
            0: bool=false, str=         "0", num=0
          "0": bool=false, str=         "0", num=0
            1: bool=true , str=         "1", num=1
          "1": bool=true , str=         "1", num=1
        "foo": bool=true , str=       "foo", num=0 (warns: numeric)
  Perl's true: bool=true , str=         "1", num=1
 Perl's false: bool=false, str=          "", num=0
 "0 but true": bool=true , str="0 but true", num=0
 "3 and true": bool=true , str="3 and true", num=3 (warns: numeric)
        "0E0": bool=true , str=       "0E0", num=0
          0E0: bool=false, str=         "0", num=0
          1E0: bool=true , str=         "1", num=1
        "1E0": bool=true , str=       "1E0", num=1