What is $1 in Perl?

What is the $1? Is that the match found for (\d+)?

$line =~ /^(\d+)\s/; 
next if(!defined($1) ) ;
$paperAnnot{开发者_运维技巧$1} = $line;

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You are right, $1 means the first capturing group, in your example that is (\d+)

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Yep! It's a group match. Seeing the next there, it's probably in a loop. However, a better way of handling what you have there would be to use a conditional and test the regex:

if ( $line =~ /^(\d+)\s/ ) {
    $paperAnnot{$1} = $line;
}

or even better, give $1 a name to make it self documenting:

if ( $line =~ /^(\d+)\s/ ) {
    my $index = $1;
    $paperAnnot{$index} = $line;
}

Also, you can find more about $1, and its brethren in perldoc perlvar.

And now in Perl 5.10 and newer, you can use named capture groups:

use 5.010; # or newer
...
if ( $line =~ /^(?<linenum>\d+)\s/ ) {
    $paperAnnot{ $+{linenum} } = $line;
}

See more on Named Capture Groups with perldoc perlre.

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Yep, anything captured in parentheses is assigned to the $1, $2, $3... etc magic variables. If the regexp doesn't match they'll be undefined.