Close, but you can do better.

perl -MFile::Find -pi -e'BEGIN { find( sub { push @ARGV, $File::Find::
+name if /\.txt\z/i }, "." ) } s/find/replace/'
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With File::Find::Rule you write it more nicely.

perl -MFile::Find::Rule -pi -e'BEGIN { @ARGV = File::Find::Rule->name(
+ "*.txt" )->in( "." ) } s/find/replace/'
[download]

But in practice I’d use find -print0 | xargs -0 for this. Way too much work to do it in Perl.

Makeshifts last the longest.