use strict;
use warnings;
use GD::Graph::histogram;

my $input = 'distance.txt';
unless(open(INPUT, $input)) {
    die "\nCannot open input\n";
}

my @data;
while(my $line = <INPUT>) {
    chomp $line;
    push @data, sprintf ("%.1f\n", $line);
}

my $graph = new GD::Graph::histogram(400,600);
$graph->set(
        y_label         => 'Count',
        x_label         => 'Distances',
        title           => 'Histogram',
        x_labels_vertical => 1,
        bar_spacing     => 0,
        shadow_depth    => 1,
        shadowclr       => 'dred',
        transparent     => 0,
)
or warn $graph->error;

my $image = $graph->plot(\@data) or die $graph->error;
open( IMG, '>' . 'out.png' ) or die $!;
binmode IMG;
print IMG $image->png;
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Post a few lines of your input file (inside "code" tags).

Your $filename variable is undefined because you didn't pass any arguments to your "main" sub. @_ holds args passed to a sub. Also, you keep clobbering your @data when you assign it to @_ (which is empty).