If you need to reformat blocks of text with wrapping, I suggest you look at Text:Wrap. The regular expression you have now will over-split short lines:

use strict;
 
my $sequence = "This is a nice line ";
# Note: You should  use '\1' rather than '$1'.
$sequence =~ s/(\S{1,80})/\1\n/g;
print "*",$sequence,"*\n";
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This prints:

*This
 is
 a
 nice
 line
 *
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If $sequence contains nothing but solid blocks of non-space characters (genome sequences, for example), or you don't care about splitting short words, then unpack would be faster.

use strict;
 
my $sequence = "123456789012345678901234567890123456789012345678901234
+56789012345678901234567890123456";
my $len = int(length($sequence)/80);
my @seq = ($len > 0) ? unpack("(A80)$len A*",$sequence) : $sequence;
 
print "*",join("\n",@seq),"*\n";
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