my $new_string = $string =~ s/!/4/r;

Note that the  /r substitution modifier is only available with Perl versions 5.14+. Prior to that version, one can use the following trick (and using the  /g modifier is handy also):

c:\@Work\Perl\monks>perl -wMstrict -le
"print 'perl version: ', $];
 ;;
 my $string = '123! 987!';
 (my $new_string = $string) =~ s/!/bang/g;
 print qq{original '$string'   new string '$new_string'};
"
perl version: 5.008009
original '123! 987!'   new string '123bang 987bang'
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