Yeah, there is really no reason to use a prototype (the empty parentheses in the subroutine definition) here.

And also it would be better to pass your variable as an argument to the subroutine, rather than using is as a global variable, for example with something like this:

#!/usr/bin/perl
use strict;
use warnings;

my $x = 'a';
printme($x);
$x = 'b';
printme ($x);

sub printme {
   print "x: ", shift, "\n";
}
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which duly prints:

x: a
x: b
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