If you have some text data (one or more text files) containing double-quote characters, and you just want to make sure that all double-quotes are replaced by single quotes, this will do it:

while (<>) {
    tr/\"/\'/;
    print;
}
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(That could be done as a one-liner on a command line with perl -pe 'tr/\x22/\x27/' in.txt > out.txt -- using hex numbers to avoid having the quotation marks misinterpreted by the command-line shell.)

I would not recommend using this approach on a text file that contains a perl script. Consider what would happen with a code snippet like the following:

$name = "my name";
print "---$/ What is $name ? \t I won't tell.\n---";
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In that form, with double quotes, it produces:

---
 What is my name ?         I won't tell.
---
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If we convert all double-quotes to single-quotes, it won't run because of a syntax error -- until we figure out that we have to escape the apostrophe in won't with a backslash. But when that's fixed, the output we see is:

---$/ What is $name ? \t I won't tell.\n---
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Single quotes in perl scripts are used to assign literal string values, without interpolating variables or special characters like \n, \t, etc.