As liz and pg have already said reporting and debugging. But I also often need to run my scripts as daemons/services and can't see any output so need to see what is going on (Debugging). Also though, I need to run external programs with command line args which I have got from somewhere else (wrapper type things) I often do something as follows;

my $cmd = "extern_prog.exe -$myarg1 -$myarg2 -$myarg3";
if ($DEBUG >= 9) { &Update_Log($cmd); }
`$cmd`;

sub Update_Log
{
   my $msg = shift;
   #write $msg to logfile
   ...
}
[download]

type stuff, This lets me copy the exact command out of the log file (with the parameters I added) and run it from the command line to check if I have gotten any funny (un-expected) results. You will not believe how many times I have used this..

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Of all the things I've lost in my life, its my mind I miss the most.