Two problems: (1) your "local slurp" isn't local enough, and (2) I think you misread the OP's intention -- mpapet wants to write to the "deliver" process, not read from it. (And BTW, you should indent, and be careful about how line-wrapping in code blocks is handled here at PM.) 
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;

[download]
(Updated to use lexically-scoped file handle in the localized input block -- that's what will cause the file to be closed on leaving the block.)

In reply to Re^2: Newbie: Pipe/STDIN Clarification by graff
in thread Newbie: Pipe/STDIN Clarification by mpapet

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