I'm sorry but I don't understand that any better than what you originally posted, too many targets

Is this it?

#!/usr/bin/perl --
use strict;
use warnings;
use Data::Dump qw/ dd pp /;
my @stuff = (
{
    in   => q{__("Thank you for choosing {T_SITE} for your reservation
+")},
    want => q{__("Thank you for choosing [_0] for your reservation",{T
+_SITE})},
},
);
for my $test ( @stuff ){
    my( $in, $want ) = @{$test}{qw/ in want /};
    my $out = $in;
    $out =~ s{
(?:
    \Q__("\E
    (.+) # $1
    \Q")\E
)
    }{
        something( $1 );
    }xegis;
    dd({ -in, $in, -out, $out, -want, $want });
}
    


sub something {
    my( $what ) = @_;
    use vars '$fudge'; local $fudge;
    $what =~ s{
\{
(
  [^\}]+
)
\}

    }{
        $fudge = $1;
        q{[_0]}
    }sex;
    if( not defined $fudge){
    }
    
    qq{__("$what",{$fudge})};
}

__END__
[download]