First, always post full code, otherwise we HAVE TO GUESS at what you are doing wrong. In the code you posted, half of your variables are uninitialized, so I had to improvise. Second, I thought you were going to switch to grid, which is better suited for this. So that said, I've modified your code to work, but without grid, all your buttons end up in one row. Either switch to grid, or make a frame for each row, and when you make your buttons, switch to the next frame when your $counter goes over 4 or whatever. I think your original problem came from the way you setup your

qw/
  7 8 9 * n*
  4 5 6 - \
  1 2 3 + =
  0 . C     / )
{
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and it was getting an undef value in there.

I'm not going to do your project for you. The code below is a patched up version of your code, which runs. Getting the rows setup right is up to you. Switch to grid, or setup separate frames for your rows of buttons.

#!/usr/bin/perl
use Tk;
use warnings;
use strict;

my %button;

my $calc = 0;

my $mw = new MainWindow( -title => 'Calc' );
$mw->geometry("300x300");
my $topframe =
  $mw->Frame( -height => '60', -width => '300' )
  ->pack( -side => 'top', -expand => '0', -fill => 'x' );

my $btmframe =
  $mw->Frame( -height => '225', -width => '300', )
  ->pack( -side => 'left', -expand => '1', -fill => 'both' );

push my @rows,
  $btmframe->Frame()->pack( -expand => 1, -fill => 'both', -side => 't
+op' )
  for ( 0 .. 3 );

my $display = $topframe->Entry(
    -justify      => 'right',
    -state        => 'disabled',
    -textvariable => \$calc
  )->pack(
    -expand => '1',
    -fill   => 'x',
    -pady   => 30,
    -padx   => 20,
    -side   => 'right'
  );

$mw->bind( "<KeyRelease>", sub { &keypress } );
my $r = 1;

my $h = 1;
my $w = 1;
for my $i (qw/ 7 8 9 * n* 4 5 6 - \ 1 2 3 + = 0 . C / )
{
    if ( $i eq '+' || $i eq '=' ) {
        $h = 2;
    }
########LINE 52 BELOW################ 
    $button{$i} = $mw->Button(
        -text    => "$i",
        -width   => '1',
        -height  => "$h",
    #    -command => sub { &btnpress($i) } 
      )->pack(
        -expand => 1,
        -fill   => 'both',
        -padx   => 2,
        -pady   => 2,
        -side   => 'left',
        -ipadx  => 5,
        -ipady  => 5
      );

    $w++;
    $h = 1;
   if ( $w > 2 ) { $w = 0; $r++; } 
}

MainLoop;
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