I don't have the backend to really test this on at the moment, but a quick thought... try printing out your data before you get into the whole unless() thing:

# at the top of the script

use Data::Dumper;

... # later

print Dumper $input;

# or, sometimes more effective to ensure
# that each var doesn't have newlines or other
# nonsense

for (qw(to from body)){
    print "\n\n>$input->{$_}<\n\n";
}

unless (($input->{to}) && ($input->{from}) && ($input->{body}) && ($in
+put->{id})) {
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...that'll show the specific var with a '>' before the text, and a '<' immediately after it. Say $input->{to} was steveb@cpan.org, it would (ie. should) show:

>steveb@cpan.org<
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If you get no output because it's a webapp, open a file and dump the info:

open my $wfh, '>', '/home/user/debug.txt' or die $!;

for (qw(to from body)){
    print $wfh "\n\n>$input->{$_}<\n\n";
}
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Rule 1: always ensure that the data you're using is exactly as you expect to be.

Also, when dealing with a web server, check the server's error logs; they often have valuable info for wtf went sideways.