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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