You could simply loop through the object's data structure (usual caveats apply):

#!/usr/bin/perl

use strict;
use warnings;

use CSS;

my $css_data = do{ local $/; <DATA> };

my $css = CSS->new();

$css->read_string( $css_data );

foreach my $style ( @{ $css->{ styles } } ) {
    foreach my $selector ( @{ $style->{ selectors } } ) {
        print $selector->{ name } . "\n";
    }
}

__DATA__

.selector_one {
    margin-left: 1em;
}

.selector_two,
.selector_three {
    margin-left: 1em;
}
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