Regular expressions are not strings, so don't use $mail =~ "...";. Maybe you want to just use index instead of a regular expression?

If you really want to use a regular expression, don't use strings, use regular expressions:

...
    if( $mail =~ /\@/ ) {
        ...
    }
...
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Regular expressions are not strings

Right, but numbers aren't strings either.

Remember, this is Perl. If you use a string as a regexp, Perl will treat it as a regexp.

my ($local_part) = $mail =~ '^([^@]*)';
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works just fine.

Sure, but if you use double quoted strings, you forego the convenience of Perl knowing that you mean to write a single backslash when you want a backslash to appear in the regular expression.

$mail =~ /(.*?)\@/;
my $first_part = $1;       # better do some error-handling first ;-)
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But better have a look at CPAN for some nice email handling/parsing modules...

...will work only if the given string $mail = "abcdefgh\@gmail.com"; But in my case, i will be getting the input with email address as its. Not with an escape character.

The backslash is only needed to prevent the interpolation of an array @gmail in the double quoted string literal.  The actual value in $mail won't hold the backslash, just as the input values you're expecting to handle...

That said, maybe have a look at Email::Address.

Or is there any other method using substr?

Sure. You can use index to find the index of the first occurrence of @, and the use substr to extract everything before it.

split '@', 'a@b.c'
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There are more than one way to do it, not necessarily regex

For example:

my $str = 'MY Name@gmail.com';

my $name = substr $str, 0, index($str,'@');
# OR 
my ($name)  = split '@', $str;
# OR
my ($name) = $str=~m/(.*?)\@/;

print $name;
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