A solution in which the whole file is read into memory and processed in chunks:

#!/usr/bin/perl

use strict;
use warnings;

$/ = undef;

my $string = <DATA>;

while ($string =~ /(            # Capture into $1
                     ^U\d+\.\d  # A 'U' at the start of a line, and 2
                                # dot-separated numbers
                   )
                   \n\n         # 2 newlines
                   (            # Capture into $2
                     (?:
                       [^\n]    # Match anything but a newline
                       |        # or
                       \n(?!\n) # A newline, but only if not followed
                                # by another newline
                     )+
                   )
                   /mgx) {
    print "\$1: <$1>:\n\$2: \n<$2>\n\n";
}

__DATA__
U448.1

Several walls of the imperial city of Constantinople which had been fr
+eshly
rebuilt with masonry, and fifty-seven towers collapsed as a result of 
+a
violent earthquake which prevailed in various places.

U483.2

Or, this year the battle of Ocha according to others, won by Muirchert
+ach
Mac Erca and by Fergus Cerball son of Conall of Cremthann, and by Fiac
+hra
Lon son of the king of Dal Araide.
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Arjen