Since you're dealing with fixed length fields, substr works well:

$field_start  = 8;  # 0-based
$field_length = 8;

# Get current value:
$old_value = substr($s, $field_start, $field_length);
$old_value =~ s/\s+$//;

# Change the value:
$new_value = 'bla';
substr($s, $field_start, $field_length) =
   substr($new_value . (' 'x$field_length), 0, $field_length);
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And if you're converting from fields of one size to fields of another size:

foreach (@old_records)
{
   my $name = substr($_,  0, 8); $name =~ s/\s+$//;
   my $desc = substr($_,  8, 8); $desc =~ s/\s+$//;
   my $date = substr($_, 16, 8); $date =~ s/\s+$//;

   $name = ...reformat name...;
   $desc = ...reformat desc...;
   $date = ...reformat date...;

   push(@new_records, 
      sprintf("%-7s %-13s %s\n", $name, $desc, $date)
   );
}
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