/(?<=_)00001(?=[.]\w+$)/

##</code><code>##

rand(9999999)+1

##</code><code>##

s/(?<=_)00001(?=[.]\w+$)/sprintf "%07d" => rand(9999999)+1/e

##</code><code>##

$ perl -Mstrict -Mwarnings -le '
    my @test_filenames = qw{
        XYZ12345_X05_20110805_9999999_00000.TXT
        XYZ12345_X05_20110805_9999999_00001.TXT
        XYZ12345_X05_20110805_9999999_00002.TXT
        XYZ12345_X05_20110805_9999999_00001x.TXT
        XYZ12345_X05_20110805_00001_00001.TXT
        XYZ12345_X05_20110805_00001_00001xxx.TXT
    };

    for (@test_filenames) {
        s/(?<=_)00001(?=[.]\w+$)/sprintf "%07d" => rand(9999999)+1/e;
        print;
    }
'

##</code><code>##

XYZ12345_X05_20110805_9999999_00000.TXT
XYZ12345_X05_20110805_9999999_1656320.TXT
XYZ12345_X05_20110805_9999999_00002.TXT
XYZ12345_X05_20110805_9999999_00001x.TXT
XYZ12345_X05_20110805_00001_0901076.TXT
XYZ12345_X05_20110805_00001_00001xxx.TXT