Your example seems to have an error in making the \$from entry a Label instead of an Entry widget. I also rearranged them into a "proper order of appearance". The following code changes now works for me with fvwm2.

#!/usr/bin/perl
use Tk;

my $mw = MainWindow-> new();

$mw->Label(text => "To:", -justify => 'left')-> grid(-column => 1, -ro
+w => 1, -sticky => 'w');
$mw->Entry(-textvariable => \$ARGV[0])-> grid(-column => 2, -row => 1,
+ -sticky => 'w');

$mw->Label(text => "From:", -justify => 'left')-> grid(-column => 1, -
+row => 3, -sticky => 'w');
$mw->Entry(-textvariable => \$from, -justify => 'left')-> grid(-column
+ => 2, -row => 3, -sticky => 'w');

$mw->Label(text => "Message:", -justify => 'left')-> grid(-column => 1
+, -row => 5);
$mw->Entry(-textvariable => \$msg, -width => 50)-> grid(-column => 2, 
+-row => 5);

$mw->Button(-text => "Send", -command => \&sent)-> grid(-column => 1, 
+-row => 7);
$mw->Button(-text => "Cancel", -command => sub { exit})-> grid(-column
+ => 3, -row => 7);
$mw->title("Support Email");
MainLoop;
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