You have a single space before and a single space after both the strings. Is that a required effect? Notice:

my $page="nospaceLOTSnospace space LOTS space";
my $old_text = qq ( LOTS );
my $new_text = qq ( LESS );
$page =~ s/$old_text/$new_text/g;
print $page;
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... will output ...

nospaceLOTSnospace space LESS space

The first "LOTS" does not get replaced because you are replacing " LOTS " (space before and after) with " LESS "

-- termix