$ perl -le '
> $search    = q{(.*)\d{6}\.txt$};
> $replace   = q{ qq{XXXX_$1} };
> $to_modify = q{AAAA200912.txt};
> print $to_modify;
> $to_modify =~ s/$search/$replace/ee; 
> print $to_modify;'
AAAA200912.txt
XXXX_AAAA