This will identify bracketed expressions in sentence 2 and bracket them in sentence 1.

use strict;
use warnings;

my $snt1 = 'this.is.example.text';
my $snt2 = '[this.is.] [example.text]';

foreach ($snt2 =~ /\[(.+?)\]/g) {
  $snt1 =~ s/\Q$_\E/[$_]/;
}

print $snt1;
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(update: removed unnecessary if)