The first time gnuplot attempts to draw the graph, it fails because the data file is empty; because the fetch takes longer than starting gnuplots does.

Hence, gnuplot produces the error message:

gnuplot> plot '< tail -n 40 c:/test/bitgbp1.dat'  using 3:2:((delta_v(
+$2) > 0.0) ? 1 : -1 )with impulses lw 2 palette
                                                                      
+                                        ^
         warning: Skipping data file with no valid points
                                                                      
+                                               ^
         x range is invalid
[download]

It then pauses for 10 seconds before attempting to reread the file and then replot. By this time the first fetch has completed and the data is available; but gnuplot fails when it attempts to replot, because there hasn't been a previous successful plot!:

gnuplot> pause 10
{"success":false,"error":"Service temporarily unavailable"}
gnuplot>
gnuplot> reread
gnuplot>
gnuplot> replot;
         ^
         no previous plot
[download]

So now gnuplot is no longer running, but the perl script continues to periodically fetch the url and re-write the data file; but nothing is reading it.

Rearranging your code so that it waits until after the first fetch has been completed and the data written to the file before it spawns gnuplot may get you a bit further.

Note:I've changed various paths to suit my system, you'll need to set them back:

#!/usr/bin/perl -w
use LWP::Simple;
use Fcntl qw(:DEFAULT :flock);

my $interval = 30; # seconds between calls
my $url='http://www.cryptonator.com/api/ticker/btc-gbp';

# {"success":false,"error":"Service temporarily unavailable"}
sub fetch{

    print"ok3\n";
    my $replie = get($url);
    print"$replie \n";
    my($a,$b,$c,$d)= $replie  =~ /^.+?(\d+\.\d+).+?(\d+\.\d+)?.+?(\d+\
+.\d+).+?(\d+).*$/;
    $a="not defined" unless defined($a);
    $b="not defined" unless defined($b);
    $c="not defined" unless defined($c);
    $d="not defined" unless defined($d);
        open(OUT,">>","c:/test/bitgbp1.dat") or die "BIT File open err
+or $!\n";
        unless (flock(OUT, LOCK_EX|LOCK_NB))
        {
            warn "can't immediately write-lock the file ($!), blocking
+ ...";
            unless (flock(OUT, LOCK_EX))
            {
                die "can't get write-lock on numfile: $!";
            }
        }
        print OUT "$c,$a,$d\n";
        flock(OUT, LOCK_UN) or die  "cant unlock file\n";
        close(OUT);
        #print  "$c,$a,$d\n";

    alarm($interval);
};

$|++;
$SIG{TERM}=sub{print "ok1\n";alarm(0);exit;};
fetch();

my $pid = fork;

if($pid == 0) {
    my $pid1 = open(CHILD, "|-", "C:/Downloaded/gp45-winbin/gnuplot/bi
+nary/gnuplot.exe -persist ") or die "Couldn't fork: $!\n";
    local $/=undef;
    my $plot=<DATA>;
    print CHILD "$plot\n";
    close(CHILD);
    print"forking\n";
    exit;
}

$SIG{ALRM}=\&fetch;

print"ok2\n";

sleep;

__DATA__
reset
delta_v(x) = ( vD = x - old_v, old_v = x, vD)
old_v = NaN
unset key
set xtics border in scale 0,0 nomirror rotate by -90  offset character
+ 0,0  autojustify
set datafile separator ','
set bmargin at screen 0.15
#set palette defined (100 "red", 200 "green", 300 "blue")
set palette defined (-1 "red", 1 "#1B6821")
#set palette model RGB defined ( -1 'red', 1 '#006400', 0 'black' )
set cbrange [-1:1]
set grid
set yrange [*:*]
unset colorbox
set style fill solid noborder
#set xrange [1446061653-946684800:1446064653-946684800]
set xdata time
set timefmt "%s"
set format x "%H:%M"     # or anything else
set xlabel "time"
set xtics autofreq
data=system("tail -n 1 c:/test/bitgbp1.dat")
print delta_v(2)
plot '< tail -n 40 c:/test/bitgbp1.dat'  using 3:2:((delta_v($2) > 0.0
+) ? 1 : -1 )with impulses lw 2 palette
pause 10
reread
replot;
[download]

Also note I replaced 1 while 1; which is a busy loop that consumes 100% cpu of the core it runs on with sleep which consumes negligible cpu; and I ditched the completely pointless awl '{print $0}' from the tail commands.