use diagnostics as Basic debugging checklist so you can get a more verbose error message and stop trying to dereference $c, as "Mom" is not a reference

foreach $key (@a)
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${$key}[0]
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Just for clarification can you tell me what you are doing here ? push(@{${$key}[0]},1,33);

Update: changed 'indirect' to 'symbolic' as that is what I meant, but blanked out on while I was composing my post.

First off, Kudos for using the strict and warnings pragmas. They will really help you catch typos and silly errors in your script.

push(@{${$key}[0]},1,33);    
push(@{${$key}[0]},22,11);    
push(@{${$key}[1]},2,5);    
push(@{${$key}[2]},3,4);
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Ok you are trying to use indirect symbolic referencing to autovivify your hashes and that is not allowed under the strict pragma. You would probably be better off just predeclaring a single hash and use the day names as keys. Did you really want a 4 element array at index 0 and 2 element arrays elsewhere? Tk::Chart::Lines is going to complain about that. Anyway see if this clears anything up for you.

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

use Data::Dumper;
use Tk;
use Tk::Chart::Lines;

my $mw = MainWindow->new(
    -title      => 'Tk::Chart::Lines example',
    -background => 'white',
);

my $chart = $mw->Lines(
    -title  => 'My graph title',
    -xlabel => 'X Label',
    -ylabel => 'Y Label',
)->pack(qw / -fill both -expand 1 /);

my @a = qw/Mon Tue Wed Thurs/;

my %days;

foreach my $key (@a)
{
    push @{$days{$key}}, [1, 33];    
    push @{$days{$key}}, [22, 11];    
    push @{$days{$key}}, [2, 5];    
    push @{$days{$key}}, [3, 4];
}
  
my $c ='Mon';

# Add a legend to the graph
my @legends = ( 'legend 1', 'legend 2', 'legend 3' );
$chart->set_legend(
    -title       => 'Title legend',
    -data        => \@legends,
    -titlecolors => 'blue',
);

# Add help identification
$chart->set_balloon();
#print Dumper \$days{$c};

# Create the graph
$chart->plot( $days{$c} );

MainLoop;
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