Condensing what you already have, a short oneliner makes the job done:

perl -pe "s/\b([0-9]{10})\b/localtime($1)/ge" file_with_timestamps.txt
+ > file_with_dates.txt
[download]

Generally I always use scalar localtime (time) but here there is no need. The above is already quoted for windows, where it seems you are (doublequotes for the oneliner).

L*

PS I had a mistake in the above oneliner perl -pne instead of perl -pe it worked the same way ( p overtake n ?). Npw corrected.