You don't show us any code, so I won't show any code either:

• rename for renaming files
• perlre for replacing one part of text with another part of text, also perlop for the s/// operator
• glob respectively File::Glob for bsd_glob

Another approach, which I often prefer, is to output shell commands via print, which I then, after close inspection, pipe into another shell.

There's a rename (sometimes also called prename) program that's shipped with Perl. You can use it for this task:

rename "s/\.xml$/.txt" *.xml
[download]

And if you don't have it, it is on CPAN.

for N in *.xml; do
    mv $N ${N/%xml/txt}
done
[download]

 perl -e 'qx!for N in *.xml\; do  mv \$N \${N/%xml/txt}\; done!;'
[download]

Your bash script replaces filename like "xml.xml" to "txt.xml". I think you need to change the script as follows inorder to work as expected,

mv $N ${N/%xml/txt}