When you declare a variable - you are not only declaring it's name (what follows the leading sigil), it's type (scalar, hash, array, typeglob, etc), you are also declaring it's scope. When you declare a variable with my, you are restricting its scope to the nearest enclosing curly braces.

Probably what you want to do is change its current value and not actually declare it again:

my @array = qw(1 2 3);
@array = qw(one two three) if $op eq 'alpha';
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my @xxx;
@xxx = qw(one two three) if $op eq 'numbers';
@xxx = qw(four five) if $op eq 'alpha';
@xxx = qw(six ten) if $op eq 'alp';
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#!/usr/bin/perl -w
use strict;

hello('world');
goodbye('world');

sub hello {
    my $greeting = shift;
    print "hello $greeting\n";
}

sub goodbye {
    my $greeting = shift;
    print "goodbye $greeting\n";
}
[download]