See the documentation of XML::LibXML::Node under findnodes. If you follow the advice given there, your XPath will work: 
#!/usr/bin/perl
use warnings;
use strict;

use XML::LibXML;

my $file = '1832.tcx';

my $parser = XML::LibXML->new->parse_file($file);
my $xml    = XML::LibXML::XPathContext->new;                # No argum
+ent here!
$xml->registerNs('x',
                 'http://www.garmin.com/xmlschemas/TrainingCenterDatab
+ase/v2');


for my $key ($xml->findnodes('//x:Lap', $parser)) {         # Provide 
+the $parser here.
    my $string = $key->findvalue('@StartTime');             # No $pars
+er needed, since attributes are namespaceless.
    print "1\t$string\n";
}

my @X;
for my $node ($xml->findnodes('//x:Trackpoint', $parser)) { # Again, $
+parser as argument.
    my $time = $xml->findvalue('x:Time', $node);            # Context 
+specified as argument.
    push @X, $time;
}
print "@X\n";

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Update: comments.

لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: XML::LibXML and namespaces by choroba
in thread XML::LibXML and namespaces by Anonymous Monk

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