TIMTOWTDI. Used substr() and simple functions. 
#!/usr/bin/perl
use strict;
use warnings;
while (my $line = <DATA>){
  chomp($line);
  $line = "0" . $line while length($line) < 9;
  my $first = substr($line, 0, 3);
  my $second= substr($line, 3, 2);
  my $third = substr($line, 5, 4);
  my $str = $first . '-' . $second . '-' . $third;
  print $str . "\n";
}

__DATA__
458430764
453453214
462634878
453990002
462755714
631036456
466917461
467172570
454691673
258611036
42

[download]

I have added an extra check, just in case it is a smaller number and not the 9 numbers, we put "0" in front of our read $line. but know that Perl handles the numbers in the files as strings, only when you start doing mathematical operations with them, then Perl automatically handles them as numbers. You can also just print a warning and skip, like so:

if(length($line) != 9){
  warn "Bad data read (at line $.): '$line' length is not correct.\n";
  next; # skip processing this line
}
if ($line !~ /^\d+$/){
  warn "Bad data read (at line $.): '$line' not numerical.\n";
  next; # skip processing this line
}

[download]

edit: added latter check. Assuming we have positive integer numbers only (no E or negative numbers, no floats)

In reply to Re: Add - in between number in a file by FreeBeerReekingMonk
in thread Add - in between number in a file by notoperlbegineer

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