Below is my attempt. Some assumptions are made the most important is that "" "seq_name" will be one line below "StrCatDone". Also note I am a Perl newbie, so the code below is not of good quality. Any my logic is.

1. Read the file into an array.
2. Find the array index where you find: "StrCatDone" "1". Using the for(;;) loop.
3. Modify the array element of the next index hence the ++$i.
4. write the array back to the file.

use strict;
use warnings;

my $file = shift @ARGV;

open (my $fh1, $file) or die "Can't open file \"$file\", $!";

my @txt = <$fh1>;

close $fh1;

my $newtext = '"seq_name" "sequence.V6.0"';

my $i;


for ($i=0; $i <= $#txt; $i++) {
  last if $txt[$i]=~ m/"StrCatDone" "1"/;
  #print $txt[$i];
}

$txt[++$i]="$newtext\n";

open (my $fh2,">$file") or die "Can't open file \"$file\", $!";
print $fh2 @txt;
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