Just for the sake of it being a Friday afternoon, here is a version that only allows "real" numbers.

#!/usr/bin/perl
use strict;
use warnings;
use Tk;
use Regexp::Common qw/number/;

my $top = MainWindow->new();
$top->geometry('200x200');

my %entries;

for(1..4){
 $entries{$_}{'value'} ||= 0.0;
 $entries{$_}{'entry'} = $top->Entry(
     -textvariable => \$entries{$_}{'value'},
     -width        => 15,
     -validate     => 'key',
     -vcmd         => \&validate,
  )->pack;
}

MainLoop;

#have to make sure empty value has numeric context 
sub validate{
  my $val = shift;
  $val ||= 0.0;
  #get real numbers only 
  #this regex allows 2 decimal points....bad 
#  if( $val !~ /^[\d+\.]+$/ ){ return 0 } 
  if( $val !~  /^$RE{num}{real}$/ ) { return 0 }

  if (($val >= 0.0) and ($val <= 100.0)) {return 1}
    else{ return 0 }
}
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