xx 1 abc.txt
xx 2 def.txt
xx 3 ghi.txt
xx 3.5 jkl.txt
xx 4a mno.txt
xx 4b pqr.txt
...
xx 100 end.txt

xx 001 abc.txt
xx 002 def.txt
xx 003 ghi.txt
xx 003.5 jkl.txt
xx 004a mno.txt
xx 004b pqr.txt
...
xx 100 end.txt

#!/usr/bin/perl
opendir(DIR, ".");
@files = sort(grep(/txt$/i, readdir(DIR)));
closedir(DIR);

$len=0;
foreach $file (@files) {
    print $file."n";
    if ($file=~/xx (d+)(.*.txt)$/i) {
        $len=(length($1)>$len)?length($1):$len;
    }
}
foreach $file (@files) {
    if ($file=~/xx (d+)(.*.txt)$/i) {
        $newfile=sprintf("%0${len}d%s", $1, $2);
        rename("$file", "$newfile");
    }
}