Another way to do it:

my $str = "ello";
substr($str,0,0,"H");
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Cheers,
KM

my($str) = 'ello';
$str = "H$str";
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Did a quick benchmark out of curiosity:

KM:     substr($_,0,0,"H")
Ovid:    s/^(.*)$/H$1/
Shendal:  $_ .= "H".$_
turnstep: s/^/H/

Benchmark: timing 100000 iterations of KM, Ovid, Shendal, turnstep...
       KM: -1 wallclock secs ( 0.55 usr +  0.00 sys =  0.55 CPU)
     Ovid:  2 wallclock secs ( 2.20 usr +  0.00 sys =  2.20 CPU)
  Shendal:  0 wallclock secs ( 0.53 usr +  0.00 sys =  0.53 CPU)
 turnstep:  1 wallclock secs ( 0.98 usr +  0.00 sys =  0.98 CPU)
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Cheers,
KM

Regular expressions can handle this just fine as well:

my $string = "ello";
$string =~ s/^/Henry said H/;
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There are several methods to do this. Shendal has listed one. Here are a couple of others:

# Concatenation

my $str = 'ello';
$str    = 'H' . $str;

# Regex
my $str = 'ello';
$str    =~ s/^(.*)$/H$1/;
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my $str = 'ello';
my $char = 'H';

print $str, "\n";

substr($str, 0, 0, $char);

print $str, "\n";
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Just to follow-up KM and davorg, note that you can use substr() as an lvalue. It's just syntactic sugar, but I like the taste of it. 8^)

use strict;
my $str = "ello";
substr($str,0,0) = "H";
print $str, "\n";
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try this sub I wrote:

# sub signature:
#    insertXintoYatZ( X , Y , Z ) ;
sub insertXintoYatZ{ 
    my ( $X , $Y , $Z ) = @_;
    substr( $Y , $Z , -length($Y) ) = $X ;
    return $Y; 
}

$s = "The black cat climbed the green tree";
print "s = $s \n";
print 'insert     = '.($s = insertXintoYatZ( "tall ", $s , 26 ))."\n";
print "    (now, s =  '$s' ) \n";

# OUTPUT:
# s = The black cat climbed the green tree
# insert     = 'tall '
#     (now, s =  'The black cat climbed the tall green tree' )";
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I know this is kinda long winded but if u wanna insert into a string split the thing into its constituent characters and bung em in an array then u can insert whatever u want wherever u want.....

@bitsOfString = split(//,$yourString);
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Slán

Fian. Edited by davido: Added code tags.