use Config::IniFiles;

my $global_ini_data = Config::IniFiles->new( -file => "test.ini" );
my $regex           = $global_ini_data->val( 'client1', 'rename' );
my $filename        = "201306051200foobar.dat";

eval "sub vodoo { \$_[0] =~ $regex }";
vodoo ( $filename ); # this does in-place edit.

print $filename,$/;
__END__
060520.1200foobar.dat

##</code><code>##

eval "\$filename =~ $regex";
die $@ if $@;

##</code><code>##

sub makesub {
    # my ($client, $regex) = @_; update: wrong, we have 1 param
    my $regex = shift;
    my $sub = eval "sub { \$_[0] =~ $regex }";
    die $@ if $@;
    return $sub;
}

##</code><code>##

$hash{$client} = makesub($regex);
$hash{$client}->($filename); # $filename changed