Something like this will get you started.

perl -nle" print "$ARGV:($./$+[0]): $1" if m[([AN]{10,}]g;" file*
[download]

This will print lines like

filename:(10/50): ANNNANANAAN
[download]

where the first number is theline in the file and the second is the offset within the line.

For unix you need to swap "s for 's, and under Win32 you would need to add BEGIN{ @ARGV=map{ glob } @ARGV } to expand the wildcard filespec supplied on the comand line.

See perlrun for the switches used, and perlre for the regex.