In order to answer the questions I actually need to know what is the expected result, something the person asking this question missed to provide, but luckily in these examples it seemed to be obvious.

I got two snippets and ran both of them:

examples/split_on_the_fly.pl

my $str = "root:*:0:0:System Administrator:/var/root:/bin/sh";
my $indexes="0,4"; ######## HERE####

my ($username, $real_name) = (split /:/, $str)[$indexes]; ##and HERE##
print "$username\n";
print "$real_name\n";

(The HERE marks were in the code when I got it.)

$ perl split_on_the_fly.pl
root

In this case the author wanted to split the $str string and copy the fields 0 and 4. Effectively doing what I called split on the fly.

examples/array_slice_not_working.pl

my $str = "root:*:0:0:System Administrator:/var/root:/bin/sh";
my @fields = split /:/, $str;

$num="0,4";
my ($username, $real_name) = @fields[$num];

print "$username\n";
print "$real_name\n";

$ perl array_slice_not_working.pl
root

In the second example, apparently in an attempt to simplify the code and find the problem that way, the author has first created an array from the list returned by the split and then tried to use array slicing.

The solution is in another article.

Comments

my $str = "root:*:0:0:System Administrator:/var/root:/bin/sh"; my @fields = split /:/, $str;

$num="0,4"; my ($username, $real_name) = @fields[split /,/,$num];

print "$username\n"; print "$real_name\n";

This code working....

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Why would anyone try to use a string to specify the indexes? This works as expected if ones replaces the string with an array of numbers, i.e.:

my @indexes=(0,4);