After fixing the compile error (I assume you meant \b/), and running the code, you get "No match". If you want to match, you should get rid of the \b

use warnings;
use strict;

my $filename = "000776654103_01_QY_20099976_776520.txt";
my $account = "7766541";

if ($filename =~/$account/) {
 print $filename."\n";
}else{
  print "No match for $account\n";
}

__END__

000776654103_01_QY_20099976_776520.txt
[download]

See also:

• perlre