For something like this, I'd typically split on white space and pick the values out rather than creating a regex. Assuming there is always white space between all the items, the following would work with your example data.

while (<DATA>) {

    # Remove white space at end of line;
    s/\s+$//g;

    # Get length of line
    my $length = length;
    my @items  = split;

    # If line is long, want the internal items, 
    # if shorter want the last ones
    # 50 works with the example.
    if ( $length > 50 ) {
        print join( "\t", @items[ 1, $#items - 4, $#items - 3 ] ), "\n
+";
    }
    else {
        print join( "\t", @items[ 1, $#items - 1, $#items ] ), "\n";
    }
}

__DATA__
605 abc xxx        410.00 mV <   450.98 mV     < 490.00 mV
606 bcd yyy        -46.50  dB <   50.70  dB                
607 are zzz                      50.00  dB <   58.48  dB
[download]

Of course, you can capture the values of interest rather than printing them.

-albert