Text::Wrap does this.

use Text::Wrap;

$Text::Wrap::columns = 70;

print Text::Wrap::fill( '', '', join '', <DATA> );

__DATA__
This is a huge paragraph that no doubt spans quite a number of columns
+. My guess is that it may even span past 70 Columns, so we'll use the
+ nifty Text::Wrap module to limit the number of columns that this par
+agraph can take up to 70.
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You can use formats:

format =
^<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< ~~
$string
.
$string = get_some_really_long_string();
write;
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See perldoc perlform.

(Assuming you don't want paragraphs to wrap in the middle of words.)

Here's a nice little regex solution:

$paragraph =~ s/([^\n]{0,70})(?:\b\s*|\n)/$1\n/gi;
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my $max_width = 40;
$paragraph =~ s/([^\n]{0,$max_width})(?:\b\s*|\n)/$1\n/gio;
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Smart Substrings has some interesting insight. Combining that with the solution above:

$paragraph =~ s/([^\n]{50,70})(?:\b\s*|\n)/$1\n/gi;
while ( $paragraph =~ m/([^\n]{71})/ )
{
   $paragraph =~ s/([^\n]{70})([^\n])/$1\n$2/g;
}
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In other words, split lines which are more than 70 chars long and include no spaces. (Not sure the loop is necessary...)