Using join as others have suggested can give you better control of "inter-string separation", if you know what I mean, but you can do without it (and even without the  /e modifier):

c:\@Work\Perl>perl -wMstrict -le
"my $s = 'int ant aunt';
 print qq{'$s'};
 ;;
 my $n = 3;
 $s =~ s{ (int) }{ qq{$1 } x $n }xmsge;
 print qq{'$s'};
 ;;
 $s =~ s{ (ant \s+) }{$1$1}xmsg;
 print qq{'$s'};
"
'int ant aunt'
'int int int  ant aunt'
'int int int  ant ant aunt'
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