#!/usr/bin/perl -w
  use strict;
  use warnings;

  my @batch;
  my @batchinfo;
  my $done = 0;
  my $file;

  my $line;

  my $fh = *STDIN;
  my $prevfh;
  my $curIO = "STDIN";
  my $prevIO;

  while (!$done) {
  print STDOUT "] ";
    $line = <$fh>;
    if (defined($line)) {
      print $line;
      if ($line =~ /B\s+([\w\.\/\-\_]+)/) {
        $prevfh = $fh;
        $prevIO = $curIO;
        $file = $1;
        print "INFO: Opening batch file $file\n";
        if (open($fh, '<:encoding(UTF-8)', $file)) {
          push(@batch, $prevfh);
          push(@batchinfo, $prevIO);
          $curIO = $file;
        } else {
          warn "Could not open file '$file' $!";
          $fh = $prevfh;
          $curIO = $prevIO;
        }
      }
    } else { # end of file, restore if any
      if (@batch) {
        $fh = pop(@batch);
        $curIO = pop(@batchinfo);
        print STDERR "INFO: Current IO: $curIO\n";
      } else {
          die "No other input. Ending.";
          $done = 1;
      }
    }
  }


##</code><code>##

a
b
c
B tst2.bat
1
2
3

##</code><code>##

z
y
x
w

##</code><code>##

$ ./perl/tst.pl 
] m
m
] n
n
] o
o
] B tst.bat
B tst.bat
INFO: Opening batch file tst.bat
] a
] b
] c
] d
] e
] B tst2.bat
INFO: Opening batch file tst2.bat
] 1
] 2
] 3
] z
] y
] x
] w
] INFO: Current IO: tst.bat
] INFO: Current IO: STDIN
] No other input. Ending. at ./perl/tst.pl line 41, <STDIN> line 17.