Package Tie::Hash::SaveConfig;
@ISA = qw(Tie::StdHash);

use strict;
sub STORE {
    my $self = shift;
    SaveConfig();
    $self->SUPER::STORE(@_);
    # or (probably better) here:
    # SaveConfig();
}
 
sub SaveConfig {
    # do something ...
    print "Config saved\n";
}

1;