I believe that you need to pass a reference to a (Update: Scalar) variable into the -textvariable parameter for an entry widget.

use Tk;
require Tk::Entry;
require Tk::Button;

my $mw = MainWindow->new();

my($var1, $var2) = ('val1', 'val2');

my $entry1 = $mw->Entry(-textvariable=> \$var1)->pack();

my $entry2 = $mw->Entry(-textvariable => $var2)->pack();

my $but = $mw->Button(-text=>'print', -command=> sub{print "$var1, $va
+r2\n"; } )->pack();

MainLoop;

__END__
# Output upon pressing print after changeing both entry widget's:

entry1, val2
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Update: Please note that I intentioally left off the backslash on $val2 to demonstrate the different behavior when passing a simple value rather than a scalar reference.