null? you mean undef. You're supposed to show your real code, because the code you've shown works as you expect

#!/usr/bin/perl --


use Data::Dumper;
{
    my $a="[a,b,{perl=>monk}]";
    my $b=eval $a;
    warn $@ if $@;
    print Dumper($b);
}

{
    my $a="['a','b',{'perl'=>'monk'}]";
    my $b=eval $a;
    warn $@ if $@;
    print Dumper($b);
}

{
    use strict;
    use warnings;

    my $a="[a,b,{perl=>monk}]";
    my $b=eval $a;
    warn $@ if $@;
    print Dumper($b);
}

__END__
$VAR1 = [
          'a',
          'b',
          {
            'perl' => 'monk'
          }
        ];
$VAR1 = [
          'a',
          'b',
          {
            'perl' => 'monk'
          }
        ];
Bareword "a" not allowed while "strict subs" in use at (eval 3) line 1
+.
Bareword "b" not allowed while "strict subs" in use at (eval 3) line 1
+.
Bareword "monk" not allowed while "strict subs" in use at (eval 3) lin
+e 1.
$VAR1 = undef;
[download]

It only "breaks" under strict subs (my last example)