This will do what you ask.

#! perl -slw
use strict;
use threads;
use threads::shared;

$|=1;
my $data : shared;
my $flag : shared =  0;

sub watchit {
    my $file = shift;
    
    while( sleep 1 ) {
        if( -e $file ) {
            lock $data;
            open IN, '<', $file or die $!;
            $data = do{ local $/; <IN> };
            close IN;
            unlink $file;
            lock $flag;
            $flag = 1;
        }
    }
}

threads->create( \&watchit, $ARGV[ 0 ] )->detach;

my $n = 0;
while( 1 ) {
    if( $flag ) {
        lock $data;
        print "\nNew file\n'$data'\n\n";
        lock $flag; 
        $flag = 0;
        $n = 0;
    }
    else {
        printf "\rNothing happened : %d", $n++;
    }
    select( undef, undef, undef, .1 );
}

__END__
P:\test>396897 junk
Nothing happened : 355
New file
'the file 'junk' contains this junk
'


Nothing happened : 4103
New file
'the file 'junk' contains this junk
'


Nothing happened : 62Terminating on signal SIGINT(2)
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