in reply to perl/Tk Entry updating

In addition to the textvariable problem which has been pointed out, what you need to do is add a -browsecmd to make your program work. 
#!/usr/bin/perl
use strict;
use Tk;
use Tk::BrowseEntry;

my $AssignMet='\Input\0InputMeteo.txt';
my $AssignTmy='\Input\TMY2\FR-Trappes-71450.tm2';

my $type='Site';

my $simuTl = new MainWindow();

my $f0 = $simuTl->Frame();

my $l="Type de fichier m\xE9t\xE9o";

my $lb0 = $f0->BrowseEntry(
  -label      => $l,
  -choices    => ['Site','TMY2'],
  -variable   => \$type,

  -browsecmd  => \&detect ,
)->pack();

$f0->pack();

my $f1 = $simuTl->Frame();
my $l="Fichier de donn\xE9es m\xE9t\xE9o";

my $lab1 = $f1->Label(
  -text         => $l,
);


my $ent1;

if ($type eq "Site") {
  $ent1 = $f1->Entry(-text => $AssignMet);
  $lab1->pack(-side => 'left');
  $ent1->pack();
  $f1->pack();
}
else {
  $ent1 = $f1->Entry(-text => $AssignTmy);
  $lab1->pack(-side => 'left');
  $ent1->pack();
  $f1->pack();
}

MainLoop;

sub detect{

if ($type eq "Site") {
  $ent1-> configure (-text => $AssignMet);
}
 else {
    $ent1->configure(-text => $AssignTmy);
  }
}

[download]
As an oddity, I did find that your erroneous use of textvariable did work. This is somewhat bad programming, so I show the code here in the READMORE. Do not expect this switching of textvariable to be reliable, but somehow it works. 
#!/usr/bin/perl
use strict;
use Tk;
use Tk::BrowseEntry;

my $AssignMet='\Input\0InputMeteo.txt';
my $AssignTmy='\Input\TMY2\FR-Trappes-71450.tm2';

my $type='Site';

my $simuTl = new MainWindow();

my $f0 = $simuTl->Frame();

my $l="Type de fichier m\xE9t\xE9o";

my $lb0 = $f0->BrowseEntry(
  -label      => $l,
  -choices    => ['Site','TMY2'],
  -variable   => \$type,

  -browsecmd  => \&detect ,
)->pack();

$f0->pack();

my $f1 = $simuTl->Frame();
my $l="Fichier de donn\xE9es m\xE9t\xE9o";

my $lab1 = $f1->Label(
  -text         => $l,
);


my $ent1;

if ($type eq "Site") {
  $ent1 = $f1->Entry(-textvariable=>\$AssignMet);
  $lab1->pack(-side => 'left');
  $ent1->pack();
  $f1->pack();
}
else {
  $ent1 = $f1->Entry(-textvariable=>\$AssignTmy);
  $lab1->pack(-side => 'left');
  $ent1->pack();
  $f1->pack();
}


MainLoop;

sub detect{

if ($type eq "Site") {
  $ent1-> configure (-textvariable=>\$AssignMet);
}
 else {
    $ent1->configure(-textvariable=>\$AssignTmy);
  }

}

[download]
I'm not really a human, but I play one on earth.
Old Perl Programmer Haiku ................... flash japh

In Section Seekers of Perl Wisdom