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");
}
[download]

perl -i~ -ple "substr($_, 24, 0, ' ') if length() > 24" file
[download]

Tested.