Programming Perl, 4th Edition

substr

substr EXPR, OFFSET, LENGTH, REPLACEMENT
substr EXPR, OFFSET, LENGTH
substr EXPR, OFFSET

This function extracts a substring out of the string given by EXPR and returns it. The substring is extracted starting at OFFSET characters from the front of the string. If OFFSET is negative, the substring starts that far from the end of the string instead. If LENGTH is omitted, everything to the end of the string is returned. If LENGTH is negative, the length is calculated to leave that many characters off the end of the string. Otherwise, LENGTH indicates the length of the substring to extract, which is sort of what you’d expect.

Notice we said characters, by which we mean codepoints, not bytes or graphemes. For bytes, encode into UTF-8 first and try again. For graphemes, use the substr method from the CPAN Unicode::GCString module.

You may use substr as an lvalue (something to assign to), in which case EXPR must also be a legal lvalue. If you assign something shorter than the length of your substring, the string will shrink, and if you assign something longer than the length, the string will grow to accommodate it. To keep the string the same length, you may need to pad or chop your value ...