The printf function is sometimes handy when used to take a list of values and produce an output line that displays the values in controllable ways. The sprintf function is identical to printf for its arguments, but returns whatever would have been output by printf as a single string. (Think of it as "string printf.") For example, to create a string consisting of the letter X followed by a five-digit zero-padded value of $y, it's as easy as this:
$result = sprintf("X%05d",$y);
See the sprintf entry in Chapter 3 of Programming Perl, and the printf (3) manpage (if you have it) for a description of the arguments required by sprintf.