As you're reading from the commandline, you'll probably want either a way to warn of invalid keys or, perhaps, ignore them. The following does the latter.

A valid list of keys:

$ perl -Mstrict -Mwarnings -E '
    my %x = ( a => { b => { d => 4, e => 5 }, c => 3 } );
    my $y = \%x;
    for (split /\// => $ARGV[0]) {
        last unless ref $y;
        $y = $y->{$_};
    }
    say $y if defined $y;
' a/b/e
5
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A partially valid list of keys:

$ perl -Mstrict -Mwarnings -E '
    my %x = ( a => { b => { d => 4, e => 5 }, c => 3 } );
    my $y = \%x;
    for (split /\// => $ARGV[0]) {
        last unless ref $y;
        $y = $y->{$_};
    }
    say $y if defined $y;
' a/c/e
3
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An invalid list of keys:

$ perl -Mstrict -Mwarnings -E '
    my %x = ( a => { b => { d => 4, e => 5 }, c => 3 } );
    my $y = \%x;
    for (split /\// => $ARGV[0]) {
        last unless ref $y;
        $y = $y->{$_};
    }
    say $y if defined $y;
' a/d/e
[download]