One way to do it:

#!/usr/bin/perl

use strict;

my %hTmp;

while (<DATA>) {
  if (/^{(\w*)}/) {
    $hTmp{$1}++;
  }
}

for my $key (keys %hTmp) {
  if ($hTmp{$key} > 1) {
    print "The key $key is multiple\n";
  }
  else {
    print "$key\n";
  }
}

__DATA__
^C^D^V^V^A os01 0002 010101 R S 0012310002 00003466^B{IT}
R
{SOURCETAG}
0012310002
{ACCESSION}
000000
{PUBLICATION}
THE ORLANDO SENTINEL
{EDITION}
METRO
{DATE}
010101
{DATE}
010102
{TDATE}
Monday, January 1, 2001
{SECTION}
SPECIAL SECTION
{PAGE}
E2
{ZONE}
FLORIDA
{KEYWORDS}
VOLUNTEER SUPPORT
{SECTION}
SPECIAL SECTION1
{SEND}
YES
^C^D^V^V^A os01 0003 010101 R S 0012310003 00001558^B{IT}
R
{SOURCETAG}
0012310003
{ACCESSION}
000000
{PUBLICATION}
THE ORLANDO SENTINEL
{HI}hi
{EDITION}
METRO
{DATE}
010101
{TDATE}
Monday, January 1, 2001
{SEND}
YES
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