The following are just a few elements that trouble my understanding:

• Since you're globbing "*.txt", you'll never be trying to work on a file called _oldname_ so -- as written (or compressed?) the left side of the regex won't match, making Ln 9 an effective no-op (but one that eats CPU, nonetheless).
• Since you're globbing "*.txt" and you show no mechanism for revising the values in the regex as you cycle thru your globbed files, the assertion that this script "works fine" seems highly suspect.

Update: removed stupid comment; added below para on use of rename

Update2: jethro holds -- with good reason, ++ -- that the 'stupid remark' is the observation that _oldname_ can't match. He's right; I should have allowed for cases in the sort of format OP has now described, below; however, OP's use of glob "*.txt" would never find the .pdf files asserted in Re^2: I cannot create backup copy during file name change.
FTR, the point was not to make OP look foolish; rather, it is that one should cut'n'paste (double-checked) code into PM nodes, to avoid such goofs, rather than risk the hazards of retyping something that already exists in digital form.
(...and, BTW, the remark removed was even stupider)

Also, perldoc -f rename tells us

rename OLDNAME,NEWNAME
        Changes the name of a file; an existing file NEWNAME will be
        clobbered. Returns true for success, false otherwise.
        Behavior of this function varies wildly depending on your system
        implementation. For example, it will usually not work across...

which is (IMO) very nearly a deprecation message.

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