Does this do what you want? I have used ! instead of space to make it easier to count.

Update: Finally grasped the feature that the OP wanted and wasn't getting: In addition to substituting spaces, maintain the same number of lines. Like this:

$_ =<<'EOS';
</TABLE>

2
<PAGE>

RISK/RETURN SUMMARY AND FUND EXPENSES


PRIME MONEY MARKET FUND
EOS

my $pat = qr/(\n\s*[0-9]{1,3}\s*\n\s*<page>\s*\n\s*)(.*\n)/i;

s{$pat}{
  my ($m1, $m2) = ($1, $2);
  my $num_lines = $m1 =~ tr/\n//; # Just counting lines
  $m1 =~ tr//!/c; # Replacing all chars with ! (for visibility; use sp
+ace instead in final code)
  $m1 . $m2 . "\n" x $num_lines;
}gie or warn "No match!\n";

print;
[download]

Output is:

</TABLE>!!!!!!!!!!!!RISK/RETURN SUMMARY AND FUND EXPENSES







PRIME MONEY MARKET FUND
[download]