If you don't want to change the code in the subroutine to accumulate and return a string, instead of printing, you can try the following. Similar but prettier techniques use non-standard modules.

# somewhere near the top of your code, put this:
{
  package AccumHandle;
  sub TIEHANDLE { my( $pkg, $s ) = shift; bless \$s, $pkg }
  sub PRINT { my $self = shift; $$self .= join($,,@_).$\; }
}


# now do this where you call the subroutine:

  local ACC;
  tie *ACC, 'AccumHandle';
  select ACC;
  &log();
  select STDOUT;
  my $xx = ${tied(*ACC)};
  print "<b>$xx</x>";
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Btw - this question is a (currently unapproved) Categorized Question: How can I write a function's output to a scalar?.