The problem is most likely the \1, which is not repeating the first capture as I suspect you want. You may want to do something like:

use strict;
use warnings;

my $matchDate = qr/(\s*\d+th\s+\w+\s*)/;

while (<DATA>) {
    next unless /$matchDate\s+-\s+$matchDate/;
    print "Matched: $1 - $2\n";
}

__DATA__
5th April - 12th April
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Prints:

Matched: 5th April - 12th April
[download]

Thank u. This worked.

Thanks to this idea too -> (th|rd|st|nd)

GrandFather already identified the problem in your code; in case you'd like to extend your program to other numerals (1,2,3 - as noted by gangabass), there's another variant:

use strict;
use warnings;

my $dates = '5th April -  12th April xxxx,
ssss, 3rd May   -  23th May zzz!
1st June-2nd June,  ddd';

my $subexp = qr{
                 (?:-\s*)?           # may be we hit the second term
                 \d+ (?:st|nd|rd|th) # the day
                 \s+\w+\s*           # the month  
               }x;
     
my @hits = $dates =~ /$subexp $subexp/xg;
print join "\n", @hits;
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Regards

mwa

Not sure I'd count '23th May' as a valid date ;-)
Cheers
Chris

Don't forget to include in your pattern 1, 2 and 3 day of each month.