My preferred method:
#!/usr/bin/perl
# padding a string with leading zeros
# - $num is the number to be padded
# - $pad is the total number of digits you want in the result
$num = "345";
$pad = 5;
$paddednum = &pad_num($num, $pad);
print "num is $num, padded num is $paddednum\n";
exit(0);
##############################
sub pad_num {
# how it works:
#
# "0"x$pad yields ($pad) zeros, i.e. "0" x 5 yields "00000" ...
# this is prepended to $num, yielding a string with ($pad) zeros
# and the number. This becomes the first arg to the substr fn
#
# ($pad * -1) is the second arg to substr ... when 2nd arg to
# substr is negative, substr counts backward from the end of the string
#
# $pad is the third arg to substr
#
# thus if $pad is 5 and $num is 345,
# result is substr( "00000345", -5, 5 ) or "00345"
my ($num, $pad) = @_;
my($paddednum) = substr( (("0"x$pad).$num), ($pad*-1), $pad );
return $paddednum;
}