{
    local $/ = undef;
    $_ = <FH>;
}

##</code><code>##

     $/      The input record separator, newline by default.
             Works like awk's RS variable, including treating
             empty lines as delimiters if set to the null string.
             (Note: An empty line cannot contain any spaces or
             tabs.)  You may set it to a multi-character string
             to match a multi-character delimiter, or to undef to
             read to end of file.  Note that setting it to "\n\n"
             means something slightly different than setting it
             to "", if the file contains consecutive empty lines.
             Setting it to "" will treat two or more consecutive
             empty lines as a single empty line.  Setting it to
             "\n\n" will blindly assume that the next input
             character belongs to the next paragraph, even if
             it's a newline.  (Mnemonic: / is used to delimit
             line boundaries when quoting poetry.)

                 undef $/;
                 $_ = <FH>;          # whole file now here
                 s/\n[ \t]+/ /g;

             Remember: the value of $/ is a string, not a regexp.
             AWK has to be better for something :-)