if ( $array =~ /^\s*$/ ) { # true if line is empty or whitespace only
    ....
}

##</code><code>##

(echo SYSDATAMSG; uname -a; uptime; date) | mail -s uptime time@taproot.bz 

##</code><code>##

open(MAIL, "$file") || die "cant open spool\n";
my @mail_lines = <MAIL>;
close (MAIL);

while ( @mail_lines ) {
    $_ = shift @mail_lines;
    if ( /^SYSDATAMSG$/ ) { # next three lines are needed
        $uname_data = shift @mail_lines;
        $uptime_data = shift @mail_lines;
        $time_stamp = shift @mail_lines;
        # (maybe you want to do things with those lines before/besides printing)
        print $uname_data, $uptime_data, $time_stamp;
    }
}