The reason your attempt at substr doesn't work, this part

   while (my $elements[20] = <INFILE>) {
        substr $elements, 7, 0, ' ';
        }
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is simple, files are iterators, once you read the whole file, this part of your code  @infile=<INFILE>;

you're at the very end, there is no more file left to read

Also that is a syntax error

You probably meant to write

substr $elements[20], 7, 0, '  ';
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You ought to give perlintro another read

 if ($#ARGV != 1) {print "usage: input-file output-file \n";exit;}

In perlish perl we write that as :)

@ARGV or die "Usage: $0  input-file output-file\n";
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You can write it as  if( @ARGV  ){ die "Usage: $0  input-file output-file\n"; } if you're attached to if(){}