I'm not sure how to achieve this.

The How depends mostly on the Why - and you have not explained that. This is probably an XY Problem.

One solution is a closure.

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

my $x;

sub greeting {
    my $str = 'Hello ' . $x;
    print $str . "\n";
}

$x = 'Kevin';

greeting ();

$x = 'chickenman';

greeting ();
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