It doesn't work because you removed the 'x' the first time around.

$val = 1;

while ($val < 5) {
  $foo = "some sentence x";  # Move this here
  $foo =~ s/x/$val/;
  print "$val $foo\n";
  $val++;
}
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or

$foo = "some sentence x";
$val = 1;

while ($val < 5) {
  my $foo = $foo;  # Add this.
  $foo =~ s/x/$val/;
  print "$val $foo\n";
  $val++;
}
[download]