Now, the version that still uses Math::FFT, but PDL::Graphics::Simple only for visualisation (note the lack of needing to scale etc) - you need PDL, PDL::Graphics::Simple, and one underlying plotter, I recommend PDL::Graphics::Gnuplot:

use strict;
use warnings;
use Math::FFT;
use PDL;
use PDL::Graphics::Simple;

my $filePath = $ARGV[0] // "testData.txt";

my @data = do { open my $fh, $filePath or die "Can't open $filePath: $
+!"; local $/; split /\s+/, <$fh> };

my $fft = Math::FFT->new(\@data);
my $spectrum = $fft->spctrm;
shift @$spectrum; # Remove DC signal component

line pdl($spectrum);
print "ret> "; <STDIN>;
[download]

Even if you prefer other methods of calculation, I hope you agree that the quick visualisations available with P:G:S are a game-changer. (The last line is just so the plot window doesn't get immediately closed because your program terminated - that's not a problem if you use a REPL like perldl)