I have this fear I'm answering a homework question, but I can't resist...

@numbers = (4, 5, 6);   # not necessarly consecutive
$str = 'test number X';

my $fmt=$str;
$fmt=~s/X/\%d/;
my @newArray=map {sprintf $fmt,$_} @numbers;
print join "\n",@newArray;
[download]

By modifiying your format string to have "%d" it becomes something that sprintf can handle.

Another alternative would be:

@numbers = (4, 5, 6);   # not necessarly consecutive
$str = 'test number X';

my $fmt=$str;
$fmt=~s/X//;
my @newArray=map { "$fmt$_" } @numbers;
print join "\n",@newArray;
[download]

But that only works because your "X" is at the end of the string.
--
Mike