use warnings;
use strict;

my $out = substitute('This is a message! {\d3} --- {\d2} --- {\d3}');
print "$out\n";
exit;

sub substitute {
    my $message = shift;
    $message =~ s/\{\\d(\d+)\}/sprintf "%0${1}d", int(rand(10**$1))/ge;
    return $message;
}

__END__

This is a message! 442 --- 86 --- 885