Thanks for your helpful response almut. Thanks also for the strict lesson NetWallah.

I'm now using strict and have declared variables globally. The application works as expected.

For completion, here's the working code:

#!/usr/local/bin/perl
use Tcl::Tk;
use strict;

# Main Window
my $int = new Tcl::Tk;
my $mw = $int->mainwindow;
$mw->title("Move File");

my $frame_border_top = $mw -> Frame(-relief => "flat");
my $welcome_label = $frame_border_top -> Label(-text => "Welcome\n");

my $select_input_file_label = $mw -> Label(-text=>"Select initial file
+ for conversion:");
my $select_input_file_button = $mw -> Button(-text => "Browse", 
        -command => \&open_file
    );

my $move_button = $mw -> Button(-text => "Move File", 
        -command => \&push_button
    );

# Geometry Management

$frame_border_top -> grid(-row=>1,-column=>1,-columnspan=>2);
$welcome_label -> grid(-row=>1,-column=>1,-columnspan=>2);

$select_input_file_label -> grid(-row=>2,-column=>1);
$select_input_file_button -> grid(-row=>2,-column=>2);

$move_button -> grid(-row=>10,-column=>1,-columnspan=>2);

$int->MainLoop;

# Declare global subroutine variables

my $select_input_file;
my $selected_file;


### All sub-routines below this point. ###

# Open file dialogue

sub open_file { 
        $select_input_file = $mw-> getOpenFile(-title=>"Select File");

# These lines just to see what's going on

print "Got: $select_input_file\n";
print "My current directory is: $ENV{PWD}\n";
print "Thinking of doing:
mv ${select_input_file} -t /home/phillc\n";
}

# Push button and move file

sub push_button {

$selected_file = "mv ${select_input_file} -t /home/phillc";



system($selected_file);

}
[download]

I'd like to thank the Wisdom of the Monks for assisting with this solution.