As documented in open

use strict;
use warnings;

open my $fh, '>', \my $buffer or die "Can't open memory file: $!";
select $fh;

foo();
bar();

print STDOUT "<buffered>\n$buffer</buffered>\n";

sub foo {
    print "Hello from Foo\n";
}

sub bar {
    print "Hello from Bar\n";
}

__END__
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Update: Removed unnecessary reference to my $stdout = \*STDOUT; Thanks toolic.