So does this do what you want?

use strict;
use warnings;

my %accounts;
while ( <DATA> ) {
  push @{ $accounts{$1} }, $2 if /^(\d+)\s+(.+)/;
}

for my $k ( keys %accounts ) {
  open my $fh, '>', $k . 'txt' or die "Can't open '$k.txt': $!\n";
  print $fh join "\n", @{ $accounts{$k} };
}
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Update: Modified code slightly.