Two questions today from another valiant but ignorant novice --- with thanks in advance!

1 Given the following code, how do I use Perl's flock function to exclusively lock "file" (comprised of a list of fund tickers) so that, while subroutine "process" reads from "file" and executes, the user can't mangle "file" via subroutine "alter_file"?

2 Should "process" hang for some unknown reason, how can the user terminate whatever processs(es) UserAgent "$html_source" is executing? (Exact SIG code much appreciated.) 

use Tk;
use LWP::Simple;
use LWP::UserAgent;
use FileHandle;

$mw = MainWindow -> new;what
$screw = $mw -> Button ( -text => "screw up ticker file", 
   -command => [ \&alter_file ] ) -> pack;
$go = $mw -> Button ( -text => "go", 
   -command => [ \&process ] ) -> pack;
$stop = $mw -> Button ( -text > "stop", 
   -command => [ \&stop_process ] ) -> pack;

$html_source = new LWP::UserAgent;

MainLoop;


sub alter_file {
    open ( HANDLE1, "+< file" );  #  OPEN FOR READ/WRITE
    while ( <HANDLE1> ) {
        chop; chop; chop;  #  SCREW UP file
    }
    close HANDLE1;
}

sub process {
    open ( HANDLE2, "< file" );  #  OPEN FOR READ-ONLY

    #  WANT TO LOCK HANDLE2 SO THAT USER CAN'T SCREW UP file WHILE 
    #  THE FOLLOWING CODE EXECUTES

    while ( <HANDLE2> ) {
        $html_source -> get ( "http://quicktake.morningstar.com/
             FundNet/Snapshot.aspx?Symbol=$_" );
        #  DO WHATEVER WITH $html_source CONTENT
    }
    close HANDLE2;
}

sub stop_process {
    #  WANT TO SIGNAL USERAGENT $html_source TO STOP (OR DESTROY?) 
    #  ITSELF
    #  (AS A SIDE QUESTION: ASSUMING IT'S STILL OPEN, HOW WOULD I 
    #  THEN CLOSE HANDLE2?)
}

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In reply to Two questions on interprocess communication by cypress

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