#!/usr/bin/perl -wl

    use strict;

    my $str = 123456789012345;

    print join "-", "No", unpack("a3 a5 a7", $str);




    # Or slightly more flexible:

    # Zero pad the string to the required length
    $str = sprintf "%015s", $str;

    my $template = "###-#####-#######";
       $template =~ s/-/ /g;
       $template =~ s/(#+)/'a' . length $1/eg;


    # Template is now "a3 a5 a7"
    print join "-", "No", unpack($template, $str);




    # You could even automate it a little more as follows:
    my $digits;

    $str = 12345;

    $template =  "###-#####-#######";
    $digits   =  $template =~ tr/#//;
    $template =~ s/-/ /g;
    $template =~ s/(#+)/'a' . length $1/eg;
    $str      =  sprintf "%0*s", $digits, $str;

    print join "-", "No", unpack($template, $str);

    __END__
    Prints:

    No-123-45678-9012345
    No-123-45678-9012345
    No-000-00000-0012345