Understanding Perl's `eval` -

i trying understand perl's eval function. here test script wrote:

[red@tools-dev1 ~]$ cat evaltest.pl  #!/usr/local/bin/perl -w use strict;  while(<data>) {     chomp;     ($arg1, $arg2, $op ) = split /,/;     $cmd = "$arg1 $op $arg2";         print "$cmd\n";      $rc = eval { $cmd };     print "rc [$rc]\n"; }   __data__ 1,2,!= 1,1,!= 1,2,= 1,1,= 1,2,== 2,3,> 3,2,> 3,3,> 2,3,>= 3,2,>= 3,3,>= 2,3,< 3,2,< 3,3,< 2,3,<= 3,2,<= 3,3,<= 

when execute output ...

[red@tools-dev1 ~]$ ./evaltest.pl  1 != 2 rc [1 != 2] 1 != 1 rc [1 != 1] 1 = 2 rc [1 = 2] 1 = 1 rc [1 = 1] 1 == 2 rc [1 == 2] 2 > 3 rc [2 > 3] 3 > 2 rc [3 > 2] 3 > 3 rc [3 > 3] 2 >= 3 rc [2 >= 3] 3 >= 2 rc [3 >= 2] 3 >= 3 rc [3 >= 3] 2 < 3 rc [2 < 3] 3 < 2 rc [3 < 2] 3 < 3 rc [3 < 3] 2 <= 3 rc [2 <= 3] 3 <= 2 rc [3 <= 2] 3 <= 3 rc [3 <= 3] 

... trying output looks more this:

1 != 2 rc [1] 1 != 1 rc [0] 1 = 2 rc [0] 1 = 1 rc [1] 1 == 2 rc [0] 2 > 3 rc [0] 3 > 2 rc [1] 3 > 3 rc [0] 2 >= 3 rc [0] 3 >= 2 rc [1] 3 >= 3 rc [1] 2 < 3 rc [1] 3 < 2 rc [0] 3 < 3 rc [0] 2 <= 3 rc [1] 3 <= 2 rc [0] 3 <= 3 rc [1] 

there 2 forms of eval, , confusing them. 1 (eval block) compiles block when eval compiled; thing catch exceptions , store them in $@. looking eval string, compiles , executes perl code in given expression (storing compile time errors or run time exceptions in $@).

so eval $cmd instead of eval { $cmd }.

also note comparison operators return special boolean false value empty string in string context 0 in numeric context, output have rc [], not rc [0].

when there compile error (which happen 1 = 1 , 1 = 2), eval return undef (and mentioned, place error in $@).