Because $a and $b are special variables that are used by sort. You shouldn't really use them in your Perl code. See perldoc sort.

When reporting errors, it's usually a good idea to include the text of your error message. Also, try adding "use diagnostics" to your code to get a better description of the error.

Can't use "my $a" in sort comparison at ./sort line 7 (#1)
(F) The global variables $a and $b are reserved for sort comparisons. You mentioned $a or $b in the same line as the <=> or cmp operator, and the variable had earlier been declared as a lexical variable. Either qualify the sort variable with the package name, or rename the lexical variable.

Perl is getting confused about which $a you mean - the one that sort uses, or the one that you have declared with my. It is very bad practice to use $a or $b as a variable name, and you have just shown us why!

If you change the code to:

#!/usr/local/bin/perl
use strict;
use warnings;
my $aa = [1, 2, 5, 7, 4];
print sort { $a <=> $b } @$aa;
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it works fine with no errors or warnings.

The important thing that you didn't say was what the error was. I get 'Can't use "my $a" in sort comparison', which gives the game away somewhat!

Hello arunvelusamy,

comparing your post text with the replies (as of 11:10 UTC), I have the impression, that you altered (added to) your original post (several?) times in reactions to the commenting posts.

While this is fine, it is also less confusing for later readers to indicate so in the post in that case, i. e. marking altered/added passages as appropriate.

Thank you

While I really, really don't endorse it, you actually can use $a as your array-name, if you use a prototyped sub to sort (see sort):

#!/usr/local/bin/perl
use strict;
use warnings;
my $a = [1, 2, 5, 7, 4];
sub  mySort($$) {
        my ($a,$b) = @_;
        $a <=> $b;
}
print "", (sort mySort @$a), "\n";
__END__
Output:
12457
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Since $a is a special var in perl, you shouldn't define own vars of that name. The same applies to $b.

$a is used by the sort function to hold the first of each pair of values to be compared ($b is the second of each pair).

Now you should post your errors also, so that readers can see them, but i guess, perl didn't like you to dereference (@$a) the global variable $a that already existed before you declared my $a.