This ought to get you started, read the gnuplot docs for the commands which gnuplot will accept interactively.

#!/usr/bin/perl -w
use strict;

my $pid;
&start_gnuplot;

sub start_gnuplot{
  $pid = open(GP, "| '/usr/bin/gnuplot' 2>&1 "); 
  syswrite(GP, "plot sin(x)\n");
  sleep 2;
  syswrite(GP, "plot tan(x)\n");
  sleep 2;
  syswrite(GP, "quit\n");
}
[download]

and here is the same thing with a simple Tk frontend

#!/usr/bin/perl
use warnings;
use strict;    # echo "plot sin (x)" | gnuplot -persist 
use Tk;
use Tk::LabEntry; 
use FileHandle;

my $gnuplot_path = "/usr/bin/gnuplot";

my $top       = new MainWindow;

my $function  = "sin(x)";

my $funcentry = $top->LabEntry(
   -label        => 'Function:',
   -textvariable => \$function,
   -labelPack    => [ -side => 'left' ]
)->pack;

my $butframe = $top->Frame->pack( -anchor => 'w' );

my $plotbutton = $butframe->Button(
   -text    => 'Plot',
   -command => \&plot,
)->pack( -side => 'left' );

$butframe->Button(
   -text    => 'Quit',
   -command => \&quit,
)->pack( -side => 'left' );

$top->protocol( 'WM_DELETE_WINDOW', \&quit );

my $gnuplot = new FileHandle( "| $gnuplot_path" );
$gnuplot->autoflush( 1 );

$top->bind( "<Return>", sub { $plotbutton->invoke } );
MainLoop;

sub plot {
   $gnuplot->print( "plot $function\n" ) if $function ne '';
}

sub quit { $gnuplot->close; $top->destroy; }
[download]

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