The else clause of your if statement will never execute if ABCD immediately follows EFGH.    Try it like this instead:

open IP_FH, '<', 'test.txt' or die "Cannot open 'test.txt' $!";

my ( $match, $found );
while ( <IP_FH> ) {
    if ( /ABCD/ ... /EFGH/ ) {
        $match = 1;
        $found  .= $_;
    }
    if ( $match ) {
        $match = 0  ;
        print "========\n";
        print $found;
        $found = '';
    }
}
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