Very impressive - that's what you call lightweight - it uses around 22 kb disk space in total!

I had to hack it a bit to get it to work on Win32, and even then it behaved slightly strangely. I had to comment out:

if((getpwuid($>))[0] ne "root") {  # how is this handled on Microsoft 
+systems?
    print STDERR "You are ", (getpwuid($>))[0], " and $0 must be run a
+s root.\n";
    exit(1);
}
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as my version of Perl doesn't implement getpwuid().

Then when I ran it I got this warning:

readline() on closed filehandle main::ALIASES at smtp.pl line 77.
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. I first tried telnetting to it, then I couldn't remember all the commands (just tried HELO foo) and then quit the telnet prog).

Boy, smtp.pl didn't like that! I got tons of warnings:

Use of uninitialized value in exists at smtp.pl line 107, <GEN1> line 
+1.
Use of uninitialized value in chomp at smtp.pl line 100, <GEN1> line 1
+.
Use of uninitialized value in substitution (s///) at smtp.pl line 101,
+ <GEN1> l
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etc., you get the idea!

But, it did send e-mail. Very nice little script.