tcf03,
Your question was a little confusing to me so please forgive me if this solution doesn't do what you want. You said that the file is in the format of not having the __BEGIN__/__END__ blocks, so presumably you want to add them. You also want to process each users information before adding them. Something like the following should work

#!/usr/bin/perl
use strict;
use warnings;
{
    local $/ = "";    # paragraph mode
    while ( <DATA> ) {
        chomp;
        process_user( $_ );
        print "__BEGIN__\n$_\n__END__\n";
    }
}
sub process_user {
    my @lines = split /\n/, shift();
    for ( @lines ) {
        # do something with line;
        print "$_\n";
    }
}
__DATA__
USER 1
LOGIN date
MORE info
MORE info

USER 2
LOGIN date
MORE info
MORE info

USER3
LOGIN data
MORE info
MORE info
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