use strict; 
use warnings;

my @a=(2,4,7,5,8);

my ($position, $spaces);

# Elements have not " "  ("@a" concatenate with spaces)
if ("@a" =~ /(?:^|\ ) # Element searched maybe the first or not
            \K  # Count previous group matched within ${^PREMATCH} var
               (?{ 
                    $position=pos;
                    $spaces= () = ${^PREMATCH} =~ m, ,g;
                }) # Next group will be that which are we searching, so
                   # we take note about where is it and how many spaces
                   # should we ignore ("@a" add spaces)
            (8) # Element searched
            (?:\ |$) # Element searched maybe the last or not
            /xpg)
{
    print "Position:" ,$position - $spaces, "\n";
}