In xsh, you'd just hash the attributes:

open file.xml ;
$h := hash . //@* ;
echo { join "\n", keys $h } ;
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Similarly, in XML::LibXML:

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

use XML::LibXML;

my $xml = 'XML::LibXML'->load_xml( location => 'file.xml' );
my %h;
for ($xml->findnodes('//@*')) {
    $h{ $_->value } = 1;
}

print "$_\n" for keys %h;
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If you only want attributes of elements with no subelements, change the XPath expression to

//*[not(./*)]/@*
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Update: Not so simple in XML::Simple:

#! /usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

use XML::Simple;

my $xml = XMLin('file.xml', ForceArray   => 1,
                            KeyAttr      => [],
                            ForceContent => 1);

my %h;
@h{ map values %{ $_->[0] },
    grep 'ARRAY' eq ref,
    map values %{ $_->[0] },
    values %{ $xml->{tags}[0] }
} = ();
say for keys %h;
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($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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