#! perl -slw
use strict;
use Data::Dump qw[ pp ];

my @accts;
{
    local $/ = 'TAGEND';
    while( <DATA> ) {
        m[
            (?=.* FIRSTNAME  \s+ ( \S+ ) )
            (?=.* LASTNAME   \s+ ( \S+ ) )
            (?=.* ACCOUNT    \s+ ( \S+ ) )
            (?=.* BILLAMOUNT \s+ ( \S+ ) )
        ]xsm and push @accts, {
            firstname => $1,
            lastname => $2,
            account => $3,
            billamount => $4,
        };
    }
}

pp \@accts;

__DATA__