use strict;
use warnings;

# please note the 'sign' ">"
open( my $fh, ">", "output_file.txt" ) or die "cannot open output_file.
+txt$!";
open( my $file, "<", "/file.txt" ) or die "cannot open < file.txt $!";
while (<$file>) {
    unless (/^A/) {

        # please note "$fh" filehandle to write to
        print $fh $_;
    }
}
close $file or die $!;
close $fh   or die $!;