Here is a piece of code with comments for you:

$file = "ex1.pl";

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

foreach $line_num (0 .. $#array) {
    chomp($array[$line_num]);#prepare for m//
    if ($array[$line_num] =~ /^\s*$/) {#a blank line
        foreach (1 .. 3) {
            print $array[$line_num + $_];#remember those are not chomp
+'d
        }
        last;#without this you will see the warning about uninitialize
+d values you saw
    }
}
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