in reply to Re: First attempt at bringing in file for input/output
in thread First attempt at bringing in file for input/output

It is not allowed to optionally create a variable in an if statement.

You're probably referring to this piece of documentation (discussed in e.g. Variable Scope):

NOTE: The behaviour of a my, state, or our modified with a statement modifier conditional or loop construct (for example, my $x if ...) is undefined. The value of the my variable may be undef, any previously assigned value, or possibly anything else. Don't rely on it. Future versions of perl might do something different from the version of perl you try it out on. Here be dragons.

However, if you are referring to syntax like if (open my $fh, '<', 'file.txt'), this is perfectly legal; from Private Variables via my():

if ((my $answer = <STDIN>) =~ /^yes$/i) {
    user_agrees();
} elsif ($answer =~ /^no$/i) {
    user_disagrees();
} else {
    chomp $answer;
    die "'$answer' is neither 'yes' nor 'no'";
}

[download]
the scope of $answer extends from its declaration through the rest of that conditional, including any elsif and else clauses, but not beyond it.

One very common case of this kind of scoping being while (my $line = <>).

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Re^3: First attempt at bringing in file for input/output
by Marshall (Canon) on Oct 24, 2018 at 05:42 UTC
    I liked your post++

    To further clarify this and another thread having to do with while loop and $line=<filehande>:

    #!usr/bin/perl
use strict;
use warnings;

while (my $line =<DATA> and $line !~ /^\s*$/)
{
    print "do somehing with non blank line\n";
}  

__END__
Value of <HANDLE> construct can be "0"; test with defined() at C:\User
+s\mmtho\Documents\PerlProjects\Monks\test_myVar_inwhile.pl line 5.
Global symbol "$line" requires explicit package name (did you forget t
+o declare "my $line"?) at C:\Users\mmtho\Documents\PerlProjects\Monks
+\test_myVar_inwhile.pl line 5.
Execution of C:\Users\mmtho\Documents\PerlProjects\Monks\test_myVar_in
+while.pl aborted due to compilation errors.

    [download]
    To fix the first error message,... 
    #!usr/bin/perl
use strict;
use warnings;


while (defined (my $line =<DATA>) and $line !~ /^\s*$/)
{
    print "do somehing with non blank line\n";
}  

__END__
Global symbol "$line" requires explicit package name (did you forget t
+o declare "my $line"?) at C:\Users\mmtho\Documents\PerlProjects\Monks
+\test_myVar_inwhile_ver2.pl line 6.
Execution of C:\Users\mmtho\Documents\PerlProjects\Monks\test_myVar_in
+while_ver2.pl aborted due to compilation errors.

    [download]
    Back up and fix just this error message...
    #!usr/bin/perl
use strict;
use warnings;

my $line;

while ($line =<DATA> and $line !~ /^\s*$/)
{
    print "do somehing with non blank line\n";
}  

__END__
Value of <HANDLE> construct can be "0"; test with defined() at C:\User
+s\mmtho\Documents\PerlProjects\Monks\test_myVar_inwhile_ver2.pl line 
+7.
do somehing with non blank line
do somehing with non blank line

    [download]
    To make this work completely: 
    #!usr/bin/perl
use strict;
use warnings;

my $line;

while (defined($line =<DATA>) and $line !~ /^\s*$/)
{
    print "do somehing with non blank line\n";
}  

__END__
do somehing with non blank line
do somehing with non blank line
do somehing with non blank line
do somehing with non blank line

    [download]
[reply]
[d/l]
[select]
      To make this work completely

      In such loops I prefer to limit the scope of $line but this might not concern you.

      #!usr/bin/perl
use strict;
use warnings;

while (my $line = <DATA>) {
    last unless $line =~ /\S/;
    print "Read: $line";
}

__DATA__
do a thing with non blank line
do a thing with non blank line
last one
  
no action on this line
nor this

      [download]
[reply]
[d/l]
[select]
        I prefer to put the main condition that "ends the loop" in the conditional.
        For a command line input, that might be detection of /(Q)uit/i or something like that.
        Other secondary conditions can go in the body of the loop.
[reply]

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