1. Sustitution
We the "s" function.
Example:
--------
$sentence= "Regards and have a nice day .. ";
To replace a first occurrence of "have a nice day " by 
". Take care.." in the string $sentence, we use the 
following expression: 
$sentence =~ s/have a nice day /. Take care../;

Or omit the mach operator, use the $_ variable:
$_ = "Regards and have a nice day .. ";
And issue:
s/have a nice day /. Take care../;

The following program:

$sentence= "Regards and have a nice day .. ";
print "$sentence \n";
$sentence =~ s/ and have a nice day/. Take care/;
print "$sentence \n";
$_ = "Regards and have a nice day .. ";

s/ have a nice day/. Take care/;
print "$sentence \n";

Outputs:

Regards and have a nice day ..
Regards. Take care ..
Regards. Take care ..


If we need to make substitution of more than one 
string, we use the global option "g":
Example:
$_ = "Regards and have a nice day .. ";
print "$_\n";

s/a/A/;
print "$_\n";

s/a/A/g;
print "$_\n";

outputs:

Regards and have a nice day ..
RegArds and have a nice day ..
RegArds And hAve A nice dAy ..


------------------
To ignore case, we ust "i" option:

Example:
$_ = "Regards And have A nice day .. ";
print "$_\n";


s/A/a/g;
print "$_\n";

s/a/--/gi;
print "$_\n";

thet outputs:
Regards And have A nice day ..
Regards and have a nice day ..
Reg--rds --nd h--ve -- nice d--y ..



2. Translation
The tr function translates character-by-character. 

Example:
-------
The following statement replaces each a with x, 
each b with y, and each c with z in the variable 
$sentence. 
$sentence = "best regards .. ";
$sentence =~ tr/abc/xyz/;
print "$sentence \n";

outputs:
yest regxrds ..

The following statement returns the number of 
repeated character.

$count = ($sentence =~ tr/e/e/);
print "$count \n";
outputs 
2