Here's a happy little regex:

my $sentence = "This is a nice weather. He is a nice guy. 
We have some nice food in our dinner.";

$sentence =~ s:(\w+\s)+(\bnice\b.*?\.):A $2:g;

print "$sentence\n";
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That looks for a word followed by a space, repeated any number of times, followed by the word nice (with word boundaries around it) followed by a minimal number of characters before a final period. It replaces the first bit with 'A' and keeps the second around. /g, of course, makes it look for all the matches in the string.