If you happen to be in a bash shell, then you hardly need to invoke Perl, since you have builtins at your disposal..

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

but if you must do that in Perl you can

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

:)

Note, though, TMTOWTDI: site:perlmonks.org rename files directory


Also this: Re: What one-liners do people actually use? is awesome.

correction: /%xml/ not /xml/ good catch suhailck!