Use substr.

Something similar to this should work:

while (<>) {
  substr($_, 24, 0, ' ');
  print;
}
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Or, from the command line:

$ perl -i~ -pe "substr($_, 24, 0, ' ')" your_file_name_here
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Two bugs:

• You'll die with an substr outside of string message if there are any blank or short lines.
• You'll add the space after the newline if line is one character less than the insert point.

Fix:

while (<>) {
  chomp;
  substr($_, 24, 0, ' ') if length() > 24;
  print("$_\n");
}
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perl -i~ -ple "substr($_, 24, 0, ' ') if length() > 24" file
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Tested.

Perhaps you might wish to have a look at part - split up files according to column value.

perl -i.bak -ple 'substr($_, 25, 0, " ") if length > 24' filename.txt
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I find it odd that you add a space at the end of "1234567890123456789012345", but you don't add any spaces at the end of "123456789012345678901234".

Isn't this something you can easily do with a regex?
Something along the lines of:

my $text = ''; # put your complete works of Willie here
my $separator = '|'; # and your separator here

$text =~ s/[\^\n]([^\$\n]{25})/$1$separator/sg;

print $text;
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