$value in the foreach loop is not a real variable, it is an alias to each element in the array. As perrin said, pass the value as a parameter to the subroutine. Global variables are A Bad Thing, except when you really need them (which is not often).

By the way, things may be clearer if you:

use strict;
use warnings;

my $value = "four";
my @array = ( "one", "two", "three" );

foreach my $value (@array) 
{
    print "foreach value: $value\t";
    function();
}

sub function
{
    print "function value: $value\n";
}
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Spot the difference.