my $stringified;
{
    local $/;  # defaults to undef
    open my $fh, '<', './demo.mail';  # 3-arg open is nice
    $stringified = <$fh>;
}
# input's done, end-of-block closes the file and reverts to normal i/o

my @cmd = qw(
     /usr/lib/dovecot/deliver
     -f test@somedomain.com
     -d test@mailtask.dom );

open( CMD_OUT, '|-', @cmd ) or die $!;
print CMD_OUT $stringified;
close CMD_OUT;