A few quick comments. This code:

    if ($oids{ifName}{$a} =~ /\//) {
        @a = split(/\//, $oids{ifName}{$a});
        @b = split(/\//, $oids{ifName}{$b});
    }
    else {
        @a = $oids{ifName}{$a};
        @b = $oids{ifName}{$b};
    }
[download]

Can be replaced with simply:

    @a = split m{/}, $oids{ifName}{$a};
    @b = split m{/}, $oids{ifName}{$b};
[download]

And this code:

    if ($a[0] =~ /[A-Za-z]/) {
        $a[0] ne $b[0] ? $a[0] cmp $b[0] : ($a[1] <=> $b[1] ? $a[1] <=
+> 
+$b[1] : $a[2] <=> $b[2]);
    } else {
        $a[0] != $b[0] ? $a[0] <=> $b[0] : ($a[1] <=> $b[1] ? $a[1] <=
+> 
+$b[1] : $a[2] <=> $b[2]);
    }
[download]

Can be replaced with:

    if ($a[0] =~ /[A-Za-z]/) {
        $a[0] cmp $b[0] || $a[1] <=> $b[1];
    }
    else {
        $a[0] <=> $b[0] || $a[1] <=> $b[1];
    }
[download]

I'm sure there are other refinements, but these ones stood out to me immediately.

Update: perhaps the 2nd can even be simplified to:

    $a[0] <=> $b[0] || $a[0] cmp $b[0] || $a[1] <=> $b[1];
[download]

I think so, but I haven't tested to make sure.

Another update: I noticed I left out the 3rd level comparison, but hopefully it should be obvious how to add it in. :-)