sub substitute{ 
    my $s = shift; 
    $s =~ s[\{\\d(\d+)\}][ substr int( 1e10 + rand 1e10 ), 1, $1 ]ge; 
    return $s 
};;

print substitute( $_ ) for 'A message {d3} of {d4}';;
A message 425 of 3404

print substitute( $_ ) for 'A message {d3} of {d4}';;
A message 860 of 3797

print substitute( $_ ) for 'A message {d3} of {d4}';;
A message 453 of 2262

print substitute( $_ ) for 'A message {d3} of {d4}';;
A message 761 of 5898

print substitute( $_ ) for 'A message {d3} of {d4}';;
A message 321 of 4182

print substitute( $_ ) for 'A message {d3} of {d4}';;
A message 924 of 5054