#!/usr/bin/perl

use strict;
use warnings;

use IO::Tee ();

my $buf;

{
   open(my $buf_fh, '>', \$buf )
      or die "Unable to create a write-to-memory handle: $!\n");
   my $tee = IO::Tee->new(\*STDOUT, $buf_fh);
   my $old_select = select($tee);
   mysub();
   select($old_select); 
}

   mysub();
}

print "buffer: $buf\n";

sub mysub{
  print "mysub output\n";
}