I don't understand what you're doing. If you execute the Perl statement
    my $out = qx/grep -A 3 -B 3 xxx  file-B >> file-A/;
then  $out will be empty because output is re-directed to file-A.

c:\@Work\Perl\monks\jjmoka>dir
 Volume in drive C is Acer
 Volume Serial Number is 9480-355B

 Directory of c:\@Work\Perl\monks\jjmoka

01/20/2020  03:28 PM    <DIR>          .
01/20/2020  03:28 PM    <DIR>          ..
01/20/2020  03:22 PM                95 file-B
               1 File(s)             95 bytes
               2 Dir(s)  76,795,633,664 bytes free

c:\@Work\Perl\monks\jjmoka>type file-B
one
two
three
four
five
six
seven
eight
nine
ten
eleven
twelve
thirteen
fourteen

c:\@Work\Perl\monks\jjmoka>perl -wMstrict -MData::Dump -le
"my $out = qx/grep -A 2 -B 2 four  file-B >> file-A/;
 dd $out;
"
""

c:\@Work\Perl\monks\jjmoka>type file-A
two
three
four
five
six
--
twelve
thirteen
fourteen
[download]

This is under Win7, Perl 5.8.9.